A curate is, strictly, one entrusted with the cure or care of the souls of a parish, though in ordinary modern usage the word means an assistant clergyman, typically newly ordained.
Two senses of the word
In its strict and older sense the curate is the one who holds the cure of souls of a parish, which would make the incumbent priest himself a curate, and this sense survives in some formal usage, but in ordinary speech the word has come to mean an assistant to the incumbent, a junior clergyman who helps in the running of a parish, so the same word names both the one who holds the cure and the one who assists in it. The common usage refers to the assistant, most often a priest in the first years after ordination.
The years of formation
The assistant curacy is characteristically the first post of a newly ordained priest or deacon, a period of practical formation in parish ministry under the guidance of an experienced incumbent, so the office is closely bound up with the training and settling of the clergy in their early years. The curate enters the office by ordination and appointment as an assistant. The office is found in the Church of England and other Western churches, and it corresponds to the junior or assistant clergy generally. The curate thus most often represents the clergyman in the early stage of ministry, learning the work of a parish under supervision, though the word retains, in its strict sense, the older meaning of the one who holds the care of souls.