When you are writing HTML documents, you will find that you want Emacs to be configured in one way. You will want it to be configured in quite a different way when you are writing C++ programs or ordinary letters (that is, such as ones that you send to your mother on her birthday).
Examples of differences between the two configurations include the following:
The way text is indented. (In C++ mode, the indentation level should increase on opening braces and decrease on closing ones.)
The set of functions bound to the Ctrl+c key prefix.
The content of the menus. (C++ mode adds a menu entry called C++.)
All such configurations are saved in packages called modes. Modes exist for most programming languages, including C, C++, Java, ...