the answer is both. Cellular data networks come in all kinds of shapes and sizes. A simple one-way plan is the most obvious and inexpensive. Cellular data networks are also used for internet connectivity, and they may or may not have data roaming. In either case, the service is usually fast and reliable.
The service is often slow and unreliable, but in some cases it can be fast and reliable. In the case of DSL, the amount of data a subscriber is allowed is limited, and often the rate of data transfer is limited as well. Most cellular data plans come with a choice of speeds and package types that give a clear picture of how much data a subscriber is allowed to download (or upload) per month.