'Sitcom' is an abbreviation of 'situation comedy'. As you can see from the name, the main point of these shows is to be funny. The situation is usually based around a relationship, such as a family at home, or colleagues at work. Sitcoms have themes that follow on from one episode to the next, but usually each episode tells a complete self contained mini-story. Each episode has its own conclusion.
'Soap operas' are not comedies. They usually focus on the dramatic lives of groups of people in a particular place, and involve a range of emotions and events - love, jealousy, intrigue, betrayal, illness, death....The stories run on from one episode to the next. Each episode usually ends at a dramatic moment, so that viewers are encouraged to continue watching the series.