Formerly, esquire was a high order, ranking immediately below a knight. Although sources vary, it seems that there were traditionally five classes of esquire:
Eldest sons of knights and their eldest sons
Chiefs of ancient families (by prescription)
Esquires by creation of office (e.g., judges, QCs, JPs)
Esquires who attend teh Knight of the Bath on his installation
Nowadays, esquire is used as a courtesy for those without a higher title or religious rank with the intention, it seems, of conveying some notion of gentlemanliness. If esquire is added postnominally, the usual prefix title (Mr., Dr., etc.) is dropped.
Marshall McLuhan (1911-80)