Diffuser has the purpose to diffuse (and soften) the light. So you want to use it whenever a direct flash is to harsh. Use the search function for 'diffuser', plenty of threads and information. If that's not enough just use Google.
White card: do you refer to a tool for setting White Balance (also called gray card) or the little white card people use on top of their flash? The latter one is called Bounce Card and is a small reflector. Search function will help as well.