Most feasts are fully catered. They provide a great meal, generally presented in 2-3 courses with breaks in between for entertainments including singing dancing, chatting, etc) and to allow food to digest. As it sounds, there is normally a lot of food at a feast, so it's normal to take a small portion of each dish, and go back for more of your favourites after everyone on the table has some. And don't forget to leave some room for the next remove - the first lot of food is normally only a warm up!
One subcategory of feasting is the potluck feast, where everyone brings a plate of food, and it is presented as a buffet. Some feasts may also be a fully catered buffet, rather than food brought to the table. This is more likely to be described as a supper. Read the feast flier carefully, and you should be able to tell the difference. If not, feel free to ask the autocrat when you book.
Remember to bring your own feasting gear to a feast! Feasts also happen a lot easier if you help the autocrat out - remember everyone working there is a volunteer, and the main organisers may have been on their feet all day since early morning.