What is the difference between nonbonding and antibonding orbital?

When a bond is formed between two atoms, the atomic orbitals from each overlap. As per the Pauli exclusion principle, you cannot have two electrons in a molecule that have the same set of quantum numbers, so each atomic orbital of the component atoms splits into two molecular orbitals that are shared by the pair of atoms.

One is lower in energy than the original atomic orbitals of the component atoms, and one higher. The lower energy one is the "bonding" molecular orbital, which contributes to bonding due to its lower energy state (in relation to the component atoms' atomic orbitals). The other one, which is higher energy is the "anti-bonding" molecular orbital. If occupied, it is less stable and does not promote bonding, so in a stable molecule the electrons occupy the bonding orbital. .

A non-bonding molecular orbital, on the other hand, results when there is insufficient symmetry from the component atoms' atomic orbitals to form the necessary overlap and to increase bond order. Effectively, non bonding molecular orbitals are of the same energy as the component atoms' atomic orbitals.