Does my forum usage constitute trolling?

I've never quite understood the particulars of "trolling". I gather that it refers to people who pop in (frequently or not), drop a comment, and don't contribute much otherwise. But, just how much activity do I need to keep up in order for my contributions to be non-trollish?

For instance, I've dropped in and answered a few questions in the past, considered the matters settled, then not answered other questions for quite some time. Is this trolling behavior? And is it discouraged? If so, what if I dig up an old neglected question that deserves an answer? Is this good or bad behavior?

I suspect these answers vary between forums, but I'm still interested in the response of mods/non-mods alike.