Disc wheels are solid (or covered) wheels designed to improve airflow around the rear end of the bike and reduce drag. Used on the Track and in Triathlon, disc wheels' most widespread use is by time triallists, who prize aerodynamics above everything else in the battle against the clock.
Are disc wheels worth it for your Triathlon or TT? Here are some factors to consider when thinking about this major upgrade:
How much aero difference is there between a disc and a normal wheel? Discs are more aero than any spoked wheels, resulting in faster speeds with equivalent effort no matter how fast you ride.
Are they unmanageable in windy conditions? Not with rear discs, where your weight is underneath. Front discs are a totally different question, as they sit on your steering axis and hence are reserved for use on the track. The rule to remember is the more windy it is the less deep your front wheel should be. With a rear disc the difference in feeling under cross winds is negligible compared to a traditional wheel, and is even less so the heavier you are.
Doesn’t the extra weight slow you down on hills? Disc wheels are heavier than their spoked counterparts, no question about it. But the difference in weight is marginal compared to other areas of the bike. That said, aside from climbing the Alpe d'Huez (or a course with a series of 5%+ steep climbs), a disc will always be the fastest rear wheel. The general rule of thumb is always use a rear disc when racing in a triathlon or time trial. To save on rotational weight, go with the lightest disc you can afford.