A covalent bond is like a handshake between atoms — both partners hold on, and neither one lets go entirely. Instead of transferring electrons from one atom to another (as in ionic bonds), covalent bonding involves atoms sharing electron pairs. Each atom contributes one electron to the shared pair, and both atoms count the shared electrons as their own for the purpose of achieving a stable, filled outer shell. This type of bonding is the foundation of organic chemistry and life itself.
Consider water, one of the simplest covalent molecules. Oxygen needs two more electrons to fill its outer shell, and each hydrogen needs one. Oxygen shares one electron with each hydrogen, forming two covalent bonds. The result is the bent H-O-H molecule that makes life on Earth possible. But covalent bonds go far beyond water. Diamond is a massive network of carbon atoms, each covalently bonded to four neighbors — creating the hardest known natural material. The DNA double helix is held together by covalent bonds along its sugar-phosphate backbone. Every plastic, every medication, every protein in your body relies on covalent bonds.
Covalent bonds can be single (one shared pair, like H-H), double (two shared pairs, like O=O in oxygen gas), or triple (three shared pairs, like the N≡N bond in nitrogen gas). Triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds. Covalent bonds can also be polar (when electrons are shared unequally because one atom is more electronegative) or nonpolar (when sharing is equal). The polarity of covalent bonds in water molecules is what gives water its remarkable properties — high boiling point, surface tension, and the ability to dissolve so many substances that it is called the "universal solvent."