Request - a brief explanation of audio cards, what do they do exactly? Options

[kinben]

Can someone please provide a brief explanation as to what exactly audio cards do within a project studio setup? I have been doing audio for awhile now on my Powermac (7300/166) and have been just using the line-in on the orginal built in soundcard. What does an audio card provide? How is it integrated? Does it hook up to scsi or is it simply an interface between my mixer and the the present line-in on my soundcard? (i.e. - does the out on the audio card simply go in to the current line-in?) I am certainly up for adding one to my setup as I know they are essential for bringing one's recording 'to the next level' so to speak, but I just want to be clear on what it will do for me - this will help me figure out which one to buy as well! Any suggestions as to the latter are appreciated as well... digidesign? midiman? etc? i hope to spend under 250 US/200 quid.. (maybe used?).  Thanks for any help provided!!!