Some people do keep their younger babies in sleep suits during the day, especially if they are staying in the house. As you can see from this discussion, you aren't the only one to be confused by the different names! Manufacturers and retailers will often mix up the terms body suit, sleep suit and romper suit, which makes it hard for parents to know what, if any, differences there are.
Generally, something called a sleep suit will have long sleeves and legs, and often covered toe and fingers as well. A body suit may well be the same, but not always - they can be short sleeved or without foot covers. So there is more variety with body suits - some may be more suitable for warmer weather, or you may want to put your baby in a body suit as well as other clothing, such as a vest, bootees etc.
If you look under 'images' for both sleep suits and body suits, you will get a clearer idea of the differences.