Acid is a program that is primarily used (in its simplest versions) as a simple remix tool for amateur remixers. Its simple cut & paste, drag and drop interface allows you to lay down some beats, add a sample from your music library, cut up and change the pitch/tempo of your sample to fit with the beats.
The more advanced programs add effects, EQ, etc.
A great tool to use to get to grips with remixing and sequencing and I highly reccomend it
