In the center environment, when sectors within a given area of responsibility are slow, they can be combined so a radar team can work a larger proportion of the airspace. Controllers who are not working traffic may be on break, or they may be assigned other duties, such as reading up on new procedures, letters of agreement, general aviation notices, and the like. There's ALWAYS something to get caught up on, and much of it is required before signing on to a position.
In the olden tymes, most of us would arrive 5 to 10 minutes before the shift started, and catch up the mandatory reading, get a weather briefing, and look at the flows for Denver, so you'd have a general handle on what to expect. We'd do that BEFORE shift start.
Can't do that anymore, because of contract negotiations. FAA has to provide time-on-the-clock for required items.
expeditedescent's answer illustrates some of the differences between Tower/TRACON/Center, though. In the center (at least ZDV), you cannot have anything in the area that would be a distraction to working traffic. No TV, no books, no magazines (not even aviation related), games, etc., except FAA-issued orders and the like. If you're not assigned a position (or just coming in/going out), you normally shouldn't be in the area, unless you're doing some approved activity (discussing PTO, new procedures, etc.)