Power PC
The Power PC is, as others have mentioned, a RISC chip based on
IBM's RS/6000 chip. It is intended for use in the IBM/Apple alliance
"new" system, using Taligent's PINK OS, though it will undoubtedly
run DOS and MAC software as well (indeed, rumors of System 7 running
on the RS/6000 are at least 6 months old now).
Motorola was brought in to manufacture the chips, and possibly
improve the design (though I doubt IBM would say so).
Motorola is a full year ahead of schedule on the Power PC, while Pink
remains nebulous.
The Power PC is currently being listed in 4 different configurations. The
inital release, the 601 will NOT be as powerful as a Pentium, though it
will be faster than a 486. The later models, starting with the 603, I
believe, are slated to be significantly more powerful than a Pentium.
Because of the problems developing an object-oriented OS, it remains to
be seen where the Power PC gets used.
It is completely unrelated to either the 68K line or the 88K line.