I often notice for myself, as well as for certain other people, that depending on personality and some other subtle factors, there are types of people who (usually unintentionally) start over-explaining things during any conversation, be it a real life talk, or an online chat.
(Note: I desire to keep the scope of the question for both, to provide some general "guide" for the matter, though I admit that online superfluousness and its counterpart in real life are worthy for questions on their own.)
Over-explaining can be the source of several mild to serious issues in interpersonal communications:
becoming too detailed may lead you expose information you normally wouldn't want to and/or shouldn't (usually details on your private life);
overlong responses without focusing on the point may lead to the others becoming bored, and it distracts communication partners from contacting you for other matters in the future;
it exhausts the answerer, too, without any gain in return.
As far as I am concerned, I try limiting myself in this aspect, but sadly I'm often unable to overcome this disadvantage. For example, when I ask something from someone, I often feel compelled to explain, why I ask it, in order to avoid the impression that I have bad intentions.
What are the contributing factors that set the "proper" depth of explaining a question or a statement, and what can one do to overcome it?