Sigma and Pi Bonds
There are two types of covalent bonds: sigma (σ) and pi (π) bonds. Sigma bonds occur when s orbitals, p orbitals pointing along the axis to the other bonding atom, or hybridized orbitals collide and overlap; the region of maximum electron density is between the two bonded atoms and lies along the axis between them. Single-order bonds are always σ bonds. Sigma bonds are like taking the index finger from each hand and pointing them directly at each other, so one fingertip touches another. Just as you can rotate your fingers without losing contact, bonded atoms can rotate freely about a sigma bond.
Pi bonds occur when p orbitals (usually those left over from orbital hybridization) perpendicular to the axis of bonding overlap; in this case, the region of highest electron density is "above" or "sideways from" the axis between bonding atoms. It is important to note that hybridized or s orbitals never make π bonds. Pi bonds make up the second bond in a double bond and the second and third in a triple bond. Pi bonds are like pointing two fingers at the ceiling and moving them sideways until they touch. Unlike sigma bonds, pi bonds cannot rotate and maintain the bond; you can see this in our finger model, as turning your fingers out of alignment breaks the connection. Therefore, double or triple-bonded atoms cannot rotate relative to each other.