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 event, it starts. These events are a great way
to get a lot of "final table" tournament experience: in a single-table
sit-and-go, you start out at the final table!
A "scheduled" tournament has a specific start time announced in advance.
The poker lobby shows events of this kind coming up in the next few days
or weeks. Players sign up in advance and then arrive to play at the
scheduled time. Most of the larger events are of the scheduled type.