If you mean functions and variables in your own code - yep. Just add comments preceding the [declarations]. (for VS2013 Express, anyway)

// Here's how I use this function.
// blah-blah-blah
void MyFunction();
// this variable is used for anything you want
void useForAnyThing;

EDIT: Oops. Not definition - declaration. Changed that above.

When you mouseover MyFunction or useForAnything in your code, the comments should popup.

Works with bounded (rather than single line) comments also.

/*
This is a rather extensive
explanation for how to use this function.
Be sure to do such-and-such
And don't forget this-and-that
*/
void MyFunction();