A conversation I once had with a junior developer:

  Him: I don't write comments. My code is self-documenting.
  Me: Apparently, you have never attempted to read those documents.

Comments should include the why's of code's existence (e.g. business rules) as well as any bigger picture information about the code for usage (e.g. "This function is for X, if you are looking for the function for Y, you are probably looking for Z").