Answer:
Rabbits don't shed all the time, like cats and dogs do: instead, they go through moults. In the wild, moulting happens on a regular basis that has to do with the changing of the seasons. With pet rabbits, sometimes you see the regular, seasonal moulting you'd see in wild rabbits, but sometimes a pet rabbit's moult schedule is irregular and unexpected. Either way, generally speaking, rabbits tend to moult three to four times a year (that is, once every three to four months). Some moults are heavier than others.
It's very important to groom your rabbits daily when they're moulting (more often if they're long-haired rabbits). If rabbits consume too much fur, they will become ill (unlike cats, rabbits can't vomit, so a furball has to make its way all through the digestive system; too much fur can lead to impaction, illness, and even death). To help any small furballs moving safely through a rabbit's gut, strongly encourage hay and water when he's moulting; some people also recommend digestive enzymes (like papaya enzyme tablets, or fresh pineapple). Small furballs in a rabbit's gut are normal and usually don't pose a threat unless there are other problems (like mild dehydration).
There are many pet brushes on the market, some better than others where rabbits are concerned. Pet supplies like brushes are often reviewed on rabbit websites and blogs, so it's worthwhile to do some research. However, some people find they have more success removing loose fur with simpler methods like running wet rubber dishwashing gloves over the rabbit's body, or simply plucking (GENTLY!) loose fur out with your fingers. The main principle to keep in mind is that rabbit skin is very thin and stretchy, which makes it easier to damage or cut than cat or dog skin: always be very gentle with rabbits!