Atomic orbitals (except for s orbitals) have specific directional properties leading to different types of covalent bonds. Sigma bonds (σ bonds) are the strongest covalent bonds and are due to head-on overlapping of orbitals on two different atoms. A single bond is usually a sigma bond. Pi bonds are weaker and are due to lateral overlap between p (or d) orbitals. A double bond between two given atoms consists of one sigma and one pi bond, and a triple bond is one sigma and two pi bonds.
Pi bonds and sp2 Hybridized Orbitals - pi bonds come into play once you start forming double or triple bonds on top of the sigma bond. BECAUSE SIGMA BONDS ARE STRONGEST THEY TAKE THE MOST ENERGY TO BREAK. COMPOUNDS WITH LOTS (HEXANE, PROPANE) RELEASE LOTS OF ENERGY AS FUEL, FIRE WHEN THE BONDS ARE BROKEN!