STL is actually a subset of the full C++ standard library, which includes, for example, C headers.
The STL is the algorithms + data structures + utilities, which are templates.
From a dogmatic point of view, this might've been correct at some point in time. But, as the links from above clearly point out, most people use "STL" and "C++ standard library" interchangeably (including the very maintainers of one of the most populous C++ standard library implementations), without excluding certain parts of one or the other.