Capped DSL has a fixed amount of bandwidth depending on the option purchased, whereas Uncapped DSL allows access to an unlimited amount of data at the speed of the service ordered.
The capped service operates from a different "bandwidth pool" compared to the uncapped service. As such, the capped service is less contended than the uncapped, therefore during times of peak operation/demand, an uncapped product would perform well, but not as well as a capped product.
Capped accounts are also free of any shaping regardless of the time of day.