Venting is rather blowing off steam. Let the stress go. Complaining is more that you give an in-depth view on what you are angry about. Both can often come with good reasoning though, although venting can have good reasons, but is not necessarily so.... Of course, there's also a third category and that is whining, and then you are just saying bad things, but the only reason why is that you cannot accept things they are and are not open for any explanation.