What exactly is POSIX? I have read the Wikipedia article but I still don't understand.
POSIX is a family of standards, specified by the IEEE, to clarify and make uniform the application programming interfaces (and ancillary issues, such as command line shell utilities) provided by Unix-y operating systems.
When you write your programs to rely on POSIX standards, you can be pretty sure to be able to port them easily among a large family of Unix derivatives (including Linux, but not limited to it!); if and when you use some Linux API that's not standardized as part of POSIX, you will have a harder time if and when you want to port that program or library to other Unix-y systems (e.g., MacOSX) in the future.