You need a preamp to amplify the signal from your microphone before you can use it.
What is a Preamp?
A preamp, or preamplifier, is a device used to take a microphone-level signal and amplify it to line level. The primary reason for doing this is to bring the volume of the signal up to a level where the signal-to-noise ratio is good enough that you can record, replay, and edit the sound without introducing degradation and noise.
Effectively, all microphones in pro audio applications require a preamplifier before mixing, recording, converting to digital sound, or introducing to a PA circuit.
This refers to your basis recording signal chain in digital recording:
Mic > Preamp > A/D convertors > DAW
Sidenote: microphones transfer low-level, low-impedance sound along a balanced signal cable to enable the sound to travel long distances without signal noise being induced or loss of high frequencies. Although technically the amplifer could be included in the microphone body, this would make microphones bulky and unusable. A small head amplifier is present in most condensor mics to convert the signal to a low-Z signal to travel to the preamp.
The secondary considerations of a preamp are that it will impart "flavour" or tone to your microphone signal; judicious choice of preamp therefore is a big part of providing overall sound quality to your recording.
Common terms when referring to preamps
Transparency - refers to the preamp imparting a flavourless or neutral character. This is not undesirable.
Warmth - refers to the preamp imparting warmth to the signal; often used with respect to tube preamps.
Character - refers to any aspect of the preamps imparted tonality
Common types of preamp
Cheap vs Pro
Its an oft-quoted topic on this board that you should buy once and buy right. There exist a multitude of cheap preamps on the market that claim in the sales pitch to offer "pro-quality" sound. Good rule of thumb is that under $500, buyer beware. This is due to the high cost of quality components required to ensure that at a minimum, your sound doesn't degrade while being amplified.
There's an old example (citation needed) of the 30c capacitor - if at some point in your signal chain, all your audio is passing through one 30c capacitor made with no quality control using the cheapest possible materials - are you happy with that compromise or would you rather have a QCed $3 capacitor? Expand that example up to the full amount of components in a preamplifier and you have your cost reasoning right there.
Solid State Preamps
The most common type of preamp, using solid-state or transistor op-amps (integrated circuits) to amplify the sound.
Almost as common, a tube preamp uses vacuum tubes to amplify the mic signal. This is generally accepted to provide more warmth and a lower slew rate, plus pleasant sounding harmonic distortion resulting in a warmer, "fuzzier" character; not that this is always a good thing.
Within the preamp balanced signals must be coupled from input to internal processing to output. Common methods include transformer, capacitor, servo, and direct. A transformer-balanced preamp is often held as a desirable construction due to the sound imparted by a quality transformer, however it is in no way superior to any other type - all have their advantages.
The transformer will also present a different impedance to the microphone than a transformerless preamp which will have an impact on the sound (see variable impedance below).
This therefore is often a quick way of referring to A-Design classic following the format of a single IC op-amp preceded by a transformer - the Neve Class A transformer-coupled designs are an example of this and the design and transformer is held to be integral to the sonic character of a Neve desk.
A channel strip is a preamp which also contains some measure of dynamics or signal processing - compression, EQ, limiting, A/D conversion, digital effects etc. Channel strips can range from cheap all-in-one "prosumer" boxes to world-class boxes with a signature sound
Most preamps will have a set of features. This article ignores channel strips from here on in.
Condensor microphones require a 48v "phantom power" supply. As the mic is connected directly to the preamp, this is an obvious place to provide the power from.
Gain is another essential control, dictating by how much the original signal is boosted - different mics and sources will require different amounts of preamplification to reach nominal line leves, and these line levels themselves may be dictated by the following equipment in the signal chain, and/or digital clipping considerations.
Gain may be incremented or not, and you may have seperate gain controls for input and output stages.
Many mic preamps feature a Hi-Z DI input to allow you to plug bass guitars or other DI'd equipment directly into the pre to use its 'sonic character'
Will indicate the instant when the preamp input overloads or "clips" the pre interstage or if the output signal is likely to exceed nominal line level and clip the following device.
Offer a continual indication of the signal being fed through the preamp.
Attenuation Switching (Pads)
To reduce a "hot" input level to something more within the dynamic range of the preamp.
Again, this is a sensible and useful place to easily apply phase reversal should it be necessary.
Selectable input impedances
Modern transformerless, solid-state microphone preamps (as well as most all modern audio gear) have from 10 to 20x higher input impedance than most sources plugged into them. These are called bridging inputs, and they present nearly no load to the microphone source and certainly do not present the load the microphone was originally optimized for. A variable-impedance preamp can offer an expanded palette due to the fact that various microphones will sound different under various input impedances.