What is the difference between a sigma bond and a pi bond?

Sigma (σ) and pi (π) bonds are both types of covalent bonds formed between atoms. A sigma bond is the strongest type of covalent bond, where the atomic orbitals directly overlap between the nuclei of two atoms. It's the first bond to be formed between two atoms, and it's formed by the head-on overlap of atomic orbitals. On the other hand, a pi bond is formed by the side-on overlap of two p-orbitals. Pi bonds are typically weaker than sigma bonds, and they always occur together with a sigma bond. The p-orbitals don't overlap as much in a pi bond, and the electron density is mostly above and below the plane of the atoms. An easy way to remember this is that single bonds are sigma bonds, double bonds contain one sigma and one pi bond, and triple bonds contain one sigma and two pi bonds.