Precisely what it was like and what its use, changed with time. In the fifteenth century it might be a small spritsail-rigged warship like a cromster. Like the earlier forms of the French chaloupe, it could be a heavy and unseaworthy harbour boat or a small coastal sailing vessel. (Latterly, the chaloupe was a pulling cutter - nowadays motorized.)
Principally, and more so latterly, the hoy was a passenger and/or cargo boat. From an English point of view, it was particularly one working in the Thames Estuary and southern North Sea in the manner of the Thames sailing barge of the nineteenth and early twentieth centuries. However, the name originated in the Netherlands and from there, a slightly different vessel did the same sort of work in similar waters. In 1495, one of the Paston Letters included the phrase, An hoye of Dorderycht (a hoy of Dordrecht), in such a way as to indicate that such contact was at that date, no more than mildly unusual.
In the eighteenth and early nineteenth centuries, English hoys plied a trade between London and the north Kent coast which enabled middle class Londoners to escape the city for the more rural air of Margate, for example. This trade was much expanded by the introduction of the early steamers. This happened when the barges were taking over the cargo coasting trade on the short routes, so the hoys fell out of use.
Before the development of steam engines, the passage of boats in places like the Thames estuary and the estuaries of the Netherlands, required the skilful use of tides as much as of the wind.