This type of covalent bond is formed by the axial overlapping of half-filled atomic orbitals. The atomic orbitals overlap along the inter-nuclear axis and involve end-to-end or head-on overlap.
All the bonds in ethane are σ bonds.
This type of covalent bond is formed by the lateral or sideways overlap of the atomic orbitals. The orbital overlap takes place in such a way that their axes are parallel to each other but perpendicular to the internuclear axis.
A sigma bond is stronger than a pi bond. The reason is that the overlapping of atomic orbitals can take place to a greater extent during the formation of a sigma bond, whereas overlapping of orbitals occurs to a smaller extent during the formation of a pi bond.
A pi bond between two atoms is formed only in addition to a sigma bond. The reason is that the atoms constituting a single bond prefer to form a strong sigma bond rather than a weak pi bond.
Thus, a pi bond is always present in molecules with multiple bonds, i.e., double or triple bonds. In other words, a single bond cannot be a pi bond.
There can be free rotation of atoms around the sigma bonds. Free rotation of atoms around pi bonds is not possible because it involves breaking the pi bonds.