Well, you probably know Rebirth is a nice emulation of the Roland 303, 808 and 909. The 303 emulation is done by software algorithms, and the drum machine emulations (i.e. 808 and 909) are done using samples.

A Rebirth mod is basically a skin coupled with a set of samples for the 808 and 909. So, they still use the main Rebirth engine but they bring a new look and (possibly) some new sounds to the software.