A recurring theme of questions here on HR is about mixers. "What mixer should I choose?" "How do I hook up my mixer?" "Why doesn't my mixer sound good?" (and all sorts of variations of the above.) Anyway, I thought I'd try to set down some mixer information in one place.
The first and most important thing to know is that YOU DON'T NEED A MIXER FOR HOME RECORDING. There are some reasons why a mixer might be convenient and aid your work flow--but unless you choose the right mixer and use it properly, many of these reasons aren't valid.
REASONS YOU MIGHT WANT TO USE A MIXER
-You have multiple instruments and the mixer lets you connect them all at once without lots of patching.
-You find it more convenient to set and ride levels with a fader than in software or with little knobs on an interface.
-You are fussy about headphone monitoring during recording and like the convenience of a mixer for setting it up.
-You like having specific knobs to control headphone monitoring, pre fade listen, etc.
-You buy an expensive mixer that can work as a control surface with your DAW.
-Some mixers can replace an interface and feed USB/Firewire/Something else straight into your computer.
REASONS MAYBE YOU SHOULDN'T USE A MIXER
-Cheap ones have rubbish pre-amps that will add noise and reduce headroom.
-Routing options on cheap ones can be limiting and cause more problems than they solve.
-Cheapies don't have direct outs (which are needed to allow you to record multiple tracks).
-Cheapies don't have (or don't have enough) pre fade auxes for effective headphone monitoring set up.
So, other than looking sexy beside your lava lamp, a mixer can be more trouble than it's worth. You may well be better off with a decent interface that handles as many inputs as you need.
Still want a mixer?
THINGS TO CONSIDER WHEN CHOOSING A MIXER FOR HOME RECORDING
Work out how many sources you need and how many of these need mic pre amps (some mixers claim "16 inputs" but half of these are line level sources).
Microphone pre amps seem simple but have to do a lot. Poor ones can add extra noise (background hiss) to your recordings and also restrict the headroom you have making them prone to clipping. If that 16 channel mixer for the same price as a 2 channel interface seems too good to be true, it probably is--and the pre amps are a good place to cut costs at the manufacturer.
3. Routing to your Computer
You have a few options here. You can buy a conventional mixer and an interface with the appropriate number of inputs and outputs to connect between your mixer and computer. The interface inputs you need to connect a mixer are line level so you can save some money by not buying one with mic pre amps. Working this way, you'll need a mixer with some form of direct outs on each channel if you want to multitrack. If you just want the mixer for home recording, a mixer with the direct outs post fader is probably your best bet. If you want to record live performances and use the same mixer for the live mix, look for one with the direct out post pre amp, pre fader.
Note that it is possible to use the Insert jacks on some mixers like direct outs. This will need either a special cable (recommended) or just pushing a jack half way into the socket (always risky). Insert jacks are almost always pre fader when you consider this option...but always check.
Otherwise you can consider some of the mixers with built in USB or Firewire converters. However, BEWARE! The majority of these (at least in the economy range) only send the main mix output to the computer and also can have limitation as to what you can do with the return signal from the computer. If the return only goes to the main mix, it becomes useless for monitoring when recording. Don't laugh. A lot of cheap mixers are wired this way. Check the spec very carefully before buying--and don't trust the salesman.
At the higher end of things, consider a digital mixer. This could be a topic in its own right but, done properly, this can be a good way to convert your signals to digital then get them to your computer via ADAT, Ethernet or several other protocols.
4. Routing in the Mixer
Consider what you need when you're recording and also mixing.
When you record, you need to route the signal(s) out of your mixer into the computer but you also need to listen to any existing tracks and route these to headphones for yourself (and, potentially, everyone else in your band). If you're just doing the recording, you likely want to be able to give one or more mixes to the musicians but be able to swap between sources, mixes and anything else in your control room feed.
A good way to handle the headphone mixes is with a facility known as a "pre fade aux". You need a separate pre fade aux for each different monitor mix you want. Turn up the pre fade aux for each channel being recorded and also for any returns from the computer (the faders for these channels stay off so you don't feed them back into the mix).
Similarly, it's very useful if your mixer includes "pre fade listen" capability on each Aux output, letting you check each Aux mix at the desk.
Finally, consider how you will hook up and control your monitor speakers. Good mixers have separate monitor outs with a dedicated control so you can turn them off when recording and up again to listen to your recorded take.
5. Use as a Control Surface
Some mixers, particularly digital ones, can send and receive MIDI commands, allowing them to act a a controller for your DAW. In this mode, user buttons on you mixer can operate the transport controls (Play, Stop, etc.) on your DAW and the faders can be used to control the "virtual" faders on the software mixer in your DAW. However, this only applies to some very specific (and expensive) mixers such as some of the Yamaha digital range, some big Studers, Protools/Digidesign specifically for Protools, etc. I don't know any at the economy end of the market that have this facility. If you are buying specifically for this, be aware that there are some dedicated control surfaces (for example a Behringer BCF2000) which can do this for you--but don't actually do any mixing themselves.
Before assuming you want to use anything as a control surface, make sure your DAW is compatible! Not can even do this and not all have the right protocols for every control surface.
In summary, think carefully about whether you even need a mixer--and if you decide to go that route, check every details of the spec of the ones you're considering. All mixers are NOT created equal and very often the cheap one the guy at the music store tries to sell you will be more trouble than it's worth. My advice would be that, unless you can buy a mixer with a decent list of facilities, you're probably better off with a good interface.
Hope this helps!