I think both Nyachan and Dotty are correct.
A guest is somebody reading the forums who doesn't have an account or isn't logged into their account, hence the "2 members and 6 guests are reading this thread" stuff.
And also, if someone has been posting but then has their account deleted for some reason, any posts they have made or threads they have started will show as having been done by "guest".
That's what it seems like to me, anyway. Guests can't post as guests, but if a member becomes a guest, their posts will say "guest".