In programming, a library is a collection of pre-compiled pieces of code that can be reused in a program. Libraries simplify life for programmers, in that they provide reusable functions, routines, classes, data structures and so on which they can be reused in the programs. 

Static Libraries: 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. They are usually faster than the shared libraries because a set of commonly used object files is put into a single library executable file. One can build multiple executables without the need to recompile the file. Because it is a single file to be built, use of link commands are simpler than shared library link commands, because you specify the name of the static library.

Shared Libraries: Shared libraries are .so (or in Windows .dll, or in OS X .dylib) files. These are linked dynamically simply including the address of the library (whereas static linking is a waste of space). Dynamic linking links the libraries at the run-time. Thus, all the functions are in a special place in memory space, and every program can access them, without having multiple copies of them.