The job of a supply teacher will often vary from school to school and even class to class. Sometimes they may be working from lesson plans that have already been created by the teacher, other times they may have to come up with their own work. If they have been bought in for longer term supply, then they may have them majority of the responsibilities that the permanent teacher would have such as planning lessons and marking work.
Supple teachers are often paid more per day than permanent teachers but it may be that they only get work for a few days of the week so that money needs to last them longer. Supply teachers do not generally get paid throughout the school holidays unless they are on some sort of a contract which states they will.
Lots of teachers go in to supply teaching if they want to reduce their hours or stress levels as there is generally less responsibility as a supply teacher.
Many people say they enjoy supply teaching and find it more interesting than teaching the same class day in day out. You may need to work a little harder to gain control of the classroom though.