Technically, I have no idea what they’re called exactly. However, I do recognize them as floating hints and probably tooltips. If you haven’t seen what they look like, then you probably haven’t visited Edrei’s Footsteps in the Mirror blog.
What I like about these things is they are a great help for those who are too worried of clicking on some links. There have been surveys done where a user asks, if I clicked or submitted my information then how am I suppose to know where would it go? In comes the solution..DOM tooltips.