Wikipedia is usually great for these purposes.
RPC:
Remote procedure call (RPC) is an
Inter-process communication technology
that allows a computer program to
cause a subroutine or procedure to
execute in another address space
(commonly on another computer on a
shared network) without the programmer
explicitly coding the details for this
remote interaction.
IPC:
Inter-process communication (IPC) is a
set of techniques for the exchange of
data among multiple threads in one or
more processes. Processes may be
running on one or more computers
connected by a network.
So, RPC is just one kind of IPC.