[LHM]

So I was looking at the molecular orbital diagram of a water molecule here (it's in the middle of the page): http://www.ch.ic.ac.uk/vchemlib/course/mo_theory/
But I'm kind of confused as to why it's no longer sigma for bonding orbitals or sigma* for antibonding orbitals. Instead, it's 1a₁ or 1b₂. Are these just the same thing? And also, why is it that the 1a₁ molecular orbital of water is coming from both 2s and one of the 2p orbitals? I think this has something to do with symmetry, but how do you know how to draw molecular orbital diagram since they don't seem to be as simple as the diagrams for homonuclear diatomic molecules or heternuclear diatomic molecules?