My car does not start at times

What are the symptoms? "Doesn't start" is as descriptive as "I'm in pain, ... Why", and can have a dozen different causes. Does the engine crank at the normal speed? Do you hear any relays clicking or a rather loud single clunk each time you turn the ignition switch to "crank"? Do the head lights or interior lights stay bright or get dim when cranking the engine? How did you test the battery? Do you know how to use a digital voltmeter?