Remember the mixed tape? Well the mix tapes we know are the ones exboyfriends would make us to win our hearts back back in middle school. Yes there were those. Then there were a whole other kind of mix tapes. There were ones put out by up and comming rappers. This one was basically the way you were discovered or got any street credit. They are still made to this day (check out Drakes from last year). These tapes showed what the rapper was all about and was easily better then any full length cds.
Do yourself a favor and look at some old school hip hop mix tapes. You won't be sorry.
Also check out this 8 minute interview where one of the best NAS talks about his experiences with the mix tape.