Ridl, the signals produced by microphones are not strong enough to be effectively recorded. "mic level" output needs to be raised to "line level" somewhere in the signal chain. Some sound cards have a built in preamp (usually not the best option). (most) Mixers have multiple preamps. Usually, for home recording, a mixer is used when many signals are being mic'd separately, especially with drums, and dedicated outboard pres are used for critical sources, especially vocals and acoustic instruments. Some pres are "clean" like a power amp, they just make the signal louder. Some are more "colored" they change the sound by introducing selective distortion, and have a personal character, like a guitar amp. A Marshall amp just doesn't sound like a Fender, and a Joemeek doesn't sound like a Grace.
In general, high-end pres add more gain with less noise, and the higher end colored pres simply make stuff sound good. Older high-end pres use vacuum tubes, which add color which is often good. Recently, there has been a trend to produce pres with a small tube in the front end of the amp to introduce adjustable color. These are often called "hybrid" pres, "starved plate" or "toob/tewb" amps. Like any other product, there are better and worse versions, and the better ones tend to be more pricey.
Beware of the current myth that the mere presence of a tube will somehow make an amp better, as many of the finest amps in the world are solid state, and many of the worst are called "tube preamps" which is usually bullshit. Preamps start at about $100 and work their way uo to $5000 or even more. In general, the best pre you can afford and the best mic(s) you can afford are the single biggest hardware decision you can make to improve your recorded sound.
Personally, I'm a fan of Joemeek for modestly priced, and Avalon
for more pricey, but the pres you choose are a personal decision, like guitar amps. Some favorites of many posters here include the following:
$100 range- M Audio Audiobuddy
$200 range-Studio Projects VTB-1, M Audio DMP-3,Joemeek MQ3
$500 range- Grace 101, Joemeek VC1Qcs
Big Bucks- Anything by Avalon,Neve, Great River, and many others
Hope this helps.-Richie