Coordinate (dative) bonds: one atom brings both electrons

In an ordinary covalent bond, each atom chips in one electron to the shared pair.

In a coordinate bond, one atom brings both. The other atom just provides an empty space for them.

• A donor: an atom with a lone pair to offer (N in $\text{NH}_3$, O in $\text{H}_2\text{O}$, Cl in $\text{AlCl}_3$).
• An acceptor: an atom with an empty orbital that can take the pair ($\text{H}^{+}$, or an electron-deficient atom like Al in $\text{AlCl}_3$).

We draw a coordinate bond as an arrow pointing from the donor to the acceptor: $\text{N}\rightarrow\text{H}$.