What is σ bond, π bond?

A single bond is a $\sigma$ bond: its electron pair lies along the axis joining the two nuclei, from the head-on overlap of two orbitals. The second and third bonds of a multiple bond are $\pi$ bonds: their pairs lie on either side of the axis, from the side-on overlap of two p orbitals perpendicular to it. A double bond is one $\sigma$ and one $\pi$ bond; a triple bond one $\sigma$ and two $\pi$.