Due to shared electrons that are attracted to both nuclei (or sometimes a few nuclei, in molecules). These are the most common bonds in molecules (see Molecular physics). They are short-ranged and highly directional (anistropic). They are $\sim 30\text{ to }100 k_B T$ at room $T$.

http://www.bbc.co.uk/education/guides/z94xsbk/revision

Covalently bonded compounds

• Simple covalent molecules
• Giant covalent structures