Well, the STL (actually, that term has fallen out of favour , its now The C++ Standard Library) is a core part of the language to the extent that I've often heard it said that if you don't use it, its not really C++ at all. For example, if you use char* for strings (instead of std::string), then you're not actually using STL. If you use arrays instead of containers (std::array, std::vector etc) then its not really C++. Stuff like loops (except I guess the C++11 foreach loops), conditionals, functions etc is all basically just C. The C++ is more than just adding classes - its also adding templates, new semantics (eg move semantics, initializers etc) and the Standard Library.

Having said that, you probably DO know more C++ than you realised :-)