A marked route, which you can pay to ride as a one off thing (eta on a set day, as advertised!). Usually on private land, around 10 miles with optional jumps, and the proceeds often go to charity, or hunt supporters clubs etc.
Most people go in groups, and you decide how fast you want to go (or your horse decides
A fun ride is a set route of a certain amount of miles (e.g 10 or 12 miles), sometimes they also have XC jumps. Some you go off at a set time, others you go off when you get to the start (usually leave a few minutes if going in groups), or sometimes you all go off together. Another word for it is a Happy Hack! Its a chance to get riding where you arent allowed to normally and have a good blast. You usual pay just a set amount and then they give you a bib or a number.