Covalent Bonds
A covalent bond results when atoms share electrons to achieve the full outer layer (called an octet except with hydrogen which needs only 2 electrons to fill its outer layer). Sometimes, atoms share more than one electron. The number of atoms shared indicates the type of covalent bond. One shared atoms represents a single bond. Two shared atoms represents a double bond, and so on. Single covalent bods between atoms are quite strong, but double and triple bonds are even stronger.
Polar and Nonpolar Covalent Bonds
When the sharing of elctrons allows for a balanced molecule (as in the pictures above), the bond is said to be a nonpolar covalent bond. However, sometimes the sharing of electrons creates a molecule that is structured in a nonsymmetrical shape. This allows for opposite ends (or poles) to have opposite charges. In these cases, the bond is said to be polar. The polarity of molecules affects how they interact with other molecules - as you shall see in the water section.