A covalent bond is a type of chemical bond where two atoms share some of their electrons. An atom that shares an electron with another atom is called a covalent bonded partner. Every covalent bond has two partners, and they are in a special relationship.
Sometimes, two different atoms will be very close to each other, so close that their electrons intermix. This intermixing of electrons creates a special bond called a covalent bond.
To classify covalent bonds, we look at the number of electrons that each atom is sharing. When a bond has two electrons shared between the atoms, it's called a single bond. When four electrons are shared, it's called a double bond. When six electrons are shared, it's called a triple bond.
In some cases, atoms will share more than the typical number of electrons in a covalent bond. This is called a multiple bond, and it can have any number of electrons beyond the single, double, and triple bonds.
Covalent bonds usually make molecules very strong, which is why they are so common in many everyday materials.