It is the social rank slightly above that of gentleman, and derives from the squires who used to serve knights. Nowadays, it's synonymous with gentleman and due to its profligate use by tradesmen and the like, rarely used in modern forms of address.

To add to what Yuu said: in the US, lawyers put Esq. after their names to indicate that they are licensed attorneys

Futher to Yuu's answer, esq stands for esquire.