Preprocessor directive - the "#include " is telling the program to include code from another source, in this case iostream. I have no idea what iostream is or does, however.
Expression statements seem to function as reporting statements. They are for the benefit of the user to let him/her know what is happening.
Declaration statements are used to define things within the program. You use them to “declare” the type of object you’re using is and set its value. They are not visible to the user running the program unless reported through an expression statement.
The return statement ends the program.