In case you're curious about the history (since you said you're new to the platform), garbage collection showed up as an optional feature in Objective-C in OS X 10.5, was effectively discouraged in favor of ARC in 10.7, and officially deprecated in 10.8. Not a terribly long run, sadly. It was a nice idea that didn't work out as well in practice.
A single piece of code can potentially work in both environments, but it needs to be written with that in mind, so binaries are annotated with their GC support. Any given library or plugin can be non-GC, GC-only, or GC-optional. Non-GC libraries can only load into non-GC apps, and GC-only libraries can only load into GC apps. GC-optional libraries can load into either, but are annoying to write as I mentioned. Since the system has to support both, all system libraries had to be made GC-optional.
In addition to the usual teething bugs with the collector itself, all the libraries sprouted bugs in GC mode due to the conversion, which made garbage collection in ObjC a bit too interesting to really be nice to use.
That ends today's long, pointless, rambling comment.
