This is important to preserve that expensive paint brush you just bought. You aren't done cleaning it 'til any water squeezed out of it is clear. It takes quite a while, but your brush will last forever if you religiously clean it well after each use. Yes, you can soak it 'til you have time to clean it, but no more than overnight or the bristles will fan out. It's a firm scraper to remove loose plaster with a roller-shaped opening used to scrape off excess paint from the roller just prior to cleaning.
Rollers hold a lot of paint.
This is also a lifesaver if you run out of paint with only a little bit of wall or trim left. A one-inch wide roll should do it. Cheap tape tears when you don't want it to and stays stuck on surfaces when you want it to come off. Buy Dunn Edwards brand. Never, ever leave tape on a baseboard, doorknob, etc., for more than 24 hours after you're done painting.