What is Hybridisation?

The hybridisation of an atom is a description of its bonds by orbitals pointing along the directions predicted by VSEPR: an atom of steric number 4 is said to be sp$^3$ (four equivalent orbitals at $109.5^\circ$), of steric number 3 sp$^2$ (three orbitals at $120^\circ$ in a plane, one p orbital left perpendicular to it), of steric number 2 sp (two orbitals at $180^\circ$, two p orbitals left). The p orbitals left over form the $\pi$ bonds.