What is the difference between directives and instructions?

Directives are a signal to the pre-preprocessor that the following code must be modified prior to compilation. For instance, the #include directive tells the pre-processor that the specified file's contents must be pre-processed and then inserted at that point in the current file. The file itself should include compiler directives to prevent the file being included more than once (inclusion guards). Any line that has a leading pound symbol (#) in the first column can be considered a directive, but "using namespace..." is also a directive.

"Instructions" is an ambiguous term that we do not use in C++. Used literally, it could mean inline assembly instructions but some people might use the term to mean any C++ command. The correct terms for these are keyWords and reserved Words.