The record, publishing, merchandising, and management sides of the industry are contract crazy. In the performance arena, there are indeed contracts, but in the beginning they are more of the handshake or verbal variety, then signed contracts. When an act gets more established, they can rest assured that the written contract will be around. This does not mean that a beginning act should not try to get something in writing. The verbal contract between a club and an artist may simply be an agreement that the artist will perform on a certain date, at a certain time, for an agreed upon length of time, with what specific other act, and how much will be paid.
Many venues require some kind of written confirmation of a verbal agreement made over the phone.
This is to the advantage of the artist anyway, so it is strongly recommended to invite the situation. The name of the artist. The date, place, and time of the performance. The price of the tickets. The fee paid to the artist.