C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 37:
[37.1] What is the "STL"?

STL ("Standard Templates Library") is a library that consists mainly of (very efficient) container classes, along with some iterators and algorithms to work with the contents of these containers.

Technically speaking the term "STL" is no longer meaningful since the classes provided by the STL have been fully integrated into the standard library, along with other standard classes like std::ostream, etc. Nonetheless many people still refer to the STL as if it were a separate thing, so you might as well get used to hearing that term.