[OT]
Objective-C++ is just the compiler allowing Objective-C and C++ code in
the same translation unit. This is required if you want to call C++
from Objective-C (or vice versa) without using a bunch of extern "C"
functions as intermediaries.
[/OT]
That's pretty much it, but there are a few technical things that having
C++ instead of C as a "base language" as a result (eg. obj-c classes
only permit structure/class members when they are pod, obj-c selectors
allow reference arguments, some implementations also allow the C++ side
to use restricted pointers), and a few really annoying things that
result (sizeof(BOOL) == 1, but sizeof(bool) is machine-dependent). But
that's really going off-topic.