Sit and Go vs. scheduled tournaments

Although both are tournaments, there is a considerable difference between them.
A scheduled tournament is the classical tournaments with a predetermined start time and date, available in the poker lobby sometimes weeks before its start date. Most of these tournaments are designed for hundreds of people, feature big prize pools and can take up to several hours to finish.
The more informal Sit and Go tournaments usually consist of one or two tables and start as soon as all places are taken, as the name implies. The S&Gs shown in the poker lobby usually start within a few minutes. Offering smaller winnings due to the smaller number of players, these tournaments provide you with the unique opportunity to actually start at the final table (because there is only one). They also last significantly less.