Covalent compounds, also known as molecular compounds, are formed from the sharing of valence electrons.
These electrons are shared to fill the outermost s and p orbitals, thus stabilizing each atom in the compound.
If you examine the word, covalent, it means with valence electrons.
These compounds are formed when two non metals combine chemically.
Some common examples are water,#H_2O#, carbon dioxide, #CO_2#', and hydrogen gas which is diatomic, #H_2#.
Covalent compounds can be subdivided into polar and no polar compounds. In water, a polar molecule, the hydrogen electrons are not shared equally with the oxygen atoms; this results in a polar bond. Oxygen due to it's greater electro negativity "pulls" the electrons closer to its nucleus.
In hydrogen gas, the electronegativity is the same for both hydrogen atoms, so that there is equal sharing, making this bond a nonpolar covalent bond.