tape = CD (nowadays, usually)
mixtape can be DJ made and contain many artists mixed together, no silence parts (usually tape or mp3 mix set)
mixtape can be a compilation of different artists on a CD, with individual tracks and breaks (sold on the internet paying for cover art to bypass copyright material)
mixtape can be artist specific representation of ones original work, usually lower in sound production quality; may serve as a demo tape or a quick release to get an artist noticed