The ‘sit-and-go’ tournament is the more informal type of event. Usually these events have just one or two tables. The Poker lobby shows a list of sit-and-go events that are open for sign-ups. As soon as enough players sign up for a sit-and-go, it starts.
A scheduled tournament has a specific start time announced in advance. Players sign up in advance and then arrive to play at the scheduled time. Most of the larger events are the scheduled type.