What is the difference between a .cpp file and a .h file?

The C++ build system (compiler) knows no difference, so it's all one of conventions.

The convention is that .h files are declarations, and .cpp files are definitions.

That's why .h files are #included -- we include the declarations.

The .cpp file is the compilation unit: it's the real source code file that will be compiled (in C++).

The .h (header) files are files that will be virtually copied/pasted in the .cpp files where the #include precompiler instruction appears. Once the headers code is inserted in the .cpp code, the compilation of the .cpp can start.