Correction, there is no standard library for strictly the Objective-C language except what comes with C. Objective-C by its self (unlike C++) is a very very thin runtime built on the C language. Apple basically has their own "standard" library built with the OS that everybody uses (the NS* stuff).
You only need to worry about this if you want to compile the program for other *nix (Linux, BSD, solaris...) based OSs. If you do then you should look at GNUstep or SideStep for libraries on the *nix platforms.
(nobody cares about windows, so I won't even mention it)