A sketch is a rough or brief outline of something. If you hand in a sketch, instead of your full paper, don't expect to get a very high grade!
The word sketch can be used when talking about something visual, particularly a drawing. An artist might sketch a friend in an hour, and then go home and take weeks to complete the portrait. A sketch can also refer to a short verbal or narrative account. You might want to give your friend a brief sketch of what happened to you today, instead of boring her with every last detail, including the hours you spent washing and then folding your laundry.