The book of Joel is a record of the messages of the prophet Joel. As with most prophets, Joel’s message focuses on current events, but it also has a secondary application towards the future, some of which has occurred and some of which is yet to happen.
Little is known about Joel; when he lived is hard to pinpoint. His message is about the sorry state of the peoples’ disregard for God, which occurred when the dual disasters of locusts and drought devastated the land, representing God’s punishment. Joel then records the people’s response to it, with the book happily concluding with words of promise and hope.
Joel is sometimes called a minor prophet. This doesn’t mean he wasn’t important, but merely that the book named after him is shorter. (Compare this to the major prophets, whose books are much longer.)