In computer science, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler, linker, or binder, producing an object file and a stand-alone executable. This executable and the process of compiling it are both known as a static build of the program. Historically, libraries could only be static.
On the iPhone, static libraries have a .a extention and can only contain code. This means that any resources (xibs, images, etc…) must be packed either in a bundle or shipped separately than the library. We won't go into this part today.