What exactly is the difference? It seems like the terms can be used somewhat interchangeably, but reading the wikipedia entry for Objective-c, I came across:
In addition to C’s style of procedural programming, C++ directly supports certain forms of object-oriented programming, generic programming, and metaprogramming.
in reference to C++. So apparently they're different?
Metaprogramming, in a broad sense, means writing programs that yield other programs. E.g. like templates in C++ produce actual code only when instantiated. One can interpret a template as a program that takes a type as an input and produces an actual function/class as an output. Preprocessor is another kind of metaprogramming. Another made-up example of metaprogramming:a program that reads an XML and produces some SQL scripts according to the XML. Again, in general, a metaprogram is a program that yields another program, whereas generic programming is about parametrized(usually with other types) types(including functions) .
EDITED after considering the comments to this answer