A sigma ($\sigma$) bond is formed by the overlapping of orbitals to a larger extent along the internuclear axis, making it symmetrical around the bond-axis. Because of this larger extent of overlapping, a sigma bond is stronger as compared to a pi bond.

In contrast, a pi ($\pi$) bond is formed when atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis. This involves sidewise overlapping, resulting in two saucer-type charged clouds above and below the plane of the participating atoms. The extent of overlapping is smaller, making it weaker. Furthermore, in the formation of multiple bonds, pi bond(s) are formed in addition to a sigma bond.