The Standard Template Library (STL) is part of the C++ standard, so all implementations of C++ should support them. It includes stuff like templated containers ("vector", "list", "set", etc.), their iterators, algorithms and utilities that are used on those containers (sort, copy,etc.) , templated function objects and their associated stuff, etc. The classes in the STL are templated, meaning that they must be specialized for some specific type -- i.e. list<int>. The iostream library and the standard C library are not part of the STL since they do not involve templated types.
For a very technical reference of the STL, go here
; for a simple reference of the classes, go here