A bond A = B is formed by the interpenetration (mixing) of a half-atom orbital of atom A with a half-orbital atom of B, which have opposite spins electrons.
This interpenetration results in a molecular orbital containing a shared electronic pair.
Sigma (σ) and Pi (π) binding
Interpenetration between two atomic orbitals, resulting in a molecular obital, can occur linearly when they belong to the same axis (sigma bond) or can occur in parallel when they belong to parallel axes (pi call).
Sigma bonding is established when two atoms are bonded by a single bond, a double bond or a triple bond.
The pi bond is when two atoms bond by a double or a triple bond.
Pi bonds are dependent on sigma bonds. This means that one or two pi bonds are always accompanied by a sigma bond.
See what kind of connection is in:
Simple call = one sigma bond
Double bond = one sigma bond and one pi bond
Triple bond = two pi bonds and one sigma
A - B a bond σ
A = B one bond σ and one bond π
A ≡ B one bond σ and two bonds π