Each unit has been evaluated based on the following criteria:
Bit / Sample Rate: Measure of the quality of the A/D converter. 24 bit / 48 kHz is a good target. You can go higher, but don't go any lower than this.
Interface: Either USB 2.0 or FireWire
Mic inputs: # of preamps that accept XLR cable inputs. Combo mic/instrument inputs will accept both an XLR and "1/4 instrument cable.
Line level inputs: # of preamps that accept "1/4 instrument cable inputs.
MIDI I/O: # of MIDI Inputs / Outputs. MIDI is a one way street so these will come in pairs (1/1 for example).
TRS outputs: # of "1/4 instrument cable outputs. Can be used to route output signals to headphone amps or hardware FX.
RCA outputs: # of RCA outputs.
Bundled Software: Included DAW recording software. Note that this is usually a "lite" version and not the full suite. It is still great to start out with before making a bigger investment.