There are many different microphones out there. Something for every purpose, price, and style. This writing will not be recommending any specific brands or models. It will lay out the basic factors to consider when choosing a microphone for you needs.
While there can be a deep journey into the technical, it will be avoided here.
Generally speaking, microphones are not like the human ear. They are designed to hear or pick up different sounds in different ways. A microphone is designed to pick up some sounds better while rejecting or reducing other sounds. It is therefore important to first define what type of sound you will be working with, and then which mic will do the job.
Pattern
There are two categories of how narrow or wide a mic will pick up. "Uni directional" (one direction), and "omni directional" (more than one direction).
A uni directional is often called "Cardioid" because it's pick up pattern is shaped almost like an upside down heart. It hears mostly form the front less from the sides and least from the rear. This is the most common choice.
Others in the uni family are the "super cardioid" and "hyper cardioid." These patterns are very narrow or precise in the front and reject even more from the sides and rear. Some of these are called "shotgun mics."
An omni mic hears or picks up sounds from several or all directions. The most common omni directional mic picks up sound in a 360 degree pattern and are simply called "omni directional." Some pick up from the front and rear. These are often called "figure eight." Many times these figure eights will have a selection switch that will allow a uni, omni, or figure eight pattern. They are often called "multi pattern" mics.
Power
A "dynamic" mic has no power of it's own. It simply gets plugged in and is ready to go. A "condenser" requires a power source. Some have a built in slot for a small battery, some require a power source known as "phantom power", which is supplied by a mixer or separate power supply. Another type of power is called "plug in power" found in many small portable devices. A mic labeled "plug in power" requires this from the recorder in order to work properly.
Generally, a condenser has better sound quality than a dynamic, (generally). Condensers tend to be more delicate than the dynamic. They are also susceptible to temperature and other climate conditions and their performance will change accordingly. In a controlled environment they will usually be preferred over the dynamic. On the other hand a dynamic microphone will usually be far more robust, durable and consistent in most environments.
Proximity
Microphones either have a "proximity affect" or they don't. A proximity affect causes a rejection of material as the source gets farther away. This will help in reducing feedback from P.A.speakers. It also causes an increase in low frequency or bass as the source gets closer to the mic. We've all heard the almost unintelligible voice booming away when a person is too close to the mic. This is neither good or bad. A mic with a proximity affect has a "sweet spot" which is at a certain close distance from the source. It's sound is well balanced at this point and for this reason is preferred for vocals. If the source has a lot of movement or there are sounds to be picked up from several distances, then a mic with no or little proximity affect should be used. It's sound will be fairly even at various distances.
Frequency Response
This is the range of sound the mic will hear from the lowest (bass) to the highest sounds. If, for example, recording live music, than a "wide" frequency response will be desired. (The human hearing is 20hz to 20,000hz). If wind noise or rumble from heating and air conditioning are a problem, a mic with a "roll off" switch will be desired. This will reject the lower frequencies but allow all the rest. On the other hand, too high a frequency may cause unnatural 'S' sounds or distortion on some recordings of voices. In this case a range at around 15,000hz would be suitable. For cymbals and bird songs 20,000hz should be desired.
Dynamic Range
A microphone has it's own operating noise or "self noise". Dynamic range is the difference between loud and soft that the microphone will produce without hearing the "self noise" generated by the internal electronics. Greater range would be preferred but usually has a price attached to it. This would allow soft levels such as the gentle evening nature sounds to be heard clearly and, not get distorted at high volumes such as drums or a jet plane taking off. But if simple dialogue or non extreme levels are all that is needed even a lesser dynamic range would do the job.
Handling Noise
Handling noise occurs when vibrations outside of the microphone get passed through the casing into the "element" the part that picks up the sound. If it is to be hand held by a singer, interviewer, or nature recordist, no handling noise should be tolerated. This will cause severe bangs, pops, and other unpleasant sounds. If it is to be mounted on a floor stand, footsteps, moving of chairs etc. will travel up into the recording. If this kind of scenario is the case, look for a microphone with low handling noise. It will have an isolation design built in to separate the element from the casing. In certain situations a separate "shock mount" may be used as added isolation from the hand or stand and the mic itself.
Sensitivity
How loud is the mic, or how much does it have to be turned up to be useful. If being used in a loud environment then a lesser sensitivity will work. But if there are a lot of soft levels, such as nature or theater, then a more sensitive rating will be desired. This will give enough volume coming out of the mic to go into a PA or recording device. The rating is usually in the negative number range. The closer to zero the more sensitive it is. A desired number might be around -56db. -80db would require turning up the amplifier, mixer or recording level to possibly it's max.
Appearance
They come in all kinds of colors shapes and sizes. There will be something for everyone. Generally appearance doesn't affect functionality, you'll just pay for the extravagance.
Side Address or Front
This is the position the pick up portion of the mic is in in relation to it's casing. For example, the average hand held mic we always see, is a front address, pointing straight at the sound source. A side address is a microphone that has a body or casing that is pointed upward but the pick up portion is facing outward, toward the sound source, not upward.
Balanced or Unbalanced
Without getting real technical here, this is a wiring configuration inside the mic and the device it plugs into.
Bottom line, whenever possible, go balanced. The benefits are: less chance of electronic noises, buzzes, radio and cell phone signals coming in, generally better sound quality, and the capability of much longer cable distances, (100 or more feet). Phantom power requires a balanced mic, battery power and dynamics do not. Balanced cables must be used, as well as the device the mic will be plugged into. This all translates into higher cost but more satisfying results. Some exceptions: many small devices such as personal recorders and consumer cameras come with only "unbalanced" capability. This is because of size, cost, and short cable distance. An unbalanced mic cable should be absolutely no longer than 40 feet. Any longer and the world's problems will come your way. (You may even start picking up Pulsars from the next galaxy). If used in an environment with a lot of electronics or a big city like NYC, don't even consider unbalanced, go with balanced; or your audio nightmares will come true! If out in nature or a not too electrically busy location, unbalanced should work. One fix I'll mention here is, the use of a balancing transformer. Again, no technical jargon, just a basic; this is a little box or barrel that accepts an unbalanced mic (as close to the mic as possible) and "balances" it the rest of the way, for all the benefits mentioned above.
Connector Type
The most common connector types are: XLR (balanced), TRS (1/8 inch, 1/4 inch)(balanced), TS (1/8 inch, 1/4 inch) (unbalanced), and TRS (1/8 inch, 1/4 inch) unbalanced stereo). There are others but are not very common. (One connector gaining a lot of popularity is the USB for recording directly to a computer. Plug the USB mic into a computer and that's it). Try to keep all of your connectors as simple and compatible as possible. Avoid using adapters as much as possible. The least amount of connections, the least amount of possible problems.
Specialty or Uniqueness
There are some mics that have a special uniqueness to them. I'll cover only a few examples.
The "lavalier" or "lav". This is a very small mic usually clipped on clothing on the chest area. It is designed to pick up the voice directly and if used properly, vibrations from the chest cavity. Incorrectly attached, will produce scratching noise from rubbing on the clothing whenever the speaker moves, and will also sound weak or thin if the vibrations from the chest cavity are not picked up. Used properly will provide a natural sounding voice and give hands free mobility.
Another unique microphone is the "boundary" or "PZM". This is said to hear as the human hears. It is used in theatre or in conference rooms. It is simply placed on a large, hard, flat surface such as a table or stage floor.
A "parabolic" places a sensitive mic within a large dish. (It looks more like a satellite dish). When pointed at a distant sound, it gathers and funnels the sound into the mic. It is used most commonly on the sidelines of sports events and sometimes in nature for birding.
The "shotgun" is a very long and narrow mic designed to pick up sound only from precisely where it is aimed. I heard a story of a wedding videographer whose camera was pointed at the bride, but the shotgun mic was off just a little bit. The bride looked great but it was mom's crying that came through loud and clear.
There are more, but I chose these to show extreme differences and how they can be designed for specific rather than general situations.
Accessories
There are numerous accessories to add to or enhance the performance of the mic. Many kinds of mounts for stands, doorways, cameras etc. Windscreens to cut wind noise, foam covers to prevent "popping P's", shock mounts to reduce vibrations. If you have a need, there most probably is an invention that will meet it.
Final Thoughts
Everything costs money! Be clear about what your needs are. Most people have one or two mics that meet most of their needs. While a large arsenal would be nice, being versatile with less may be wiser, especially in the beginning. A few accessories, adapters, and adjustments will go a long way without over doing it on the purchasing.
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