Powered Monitors have an amplifier as well as speakers built-in to their cabinet. This is a great idea for several reasons, most notably:
The amplifiers are electronically matched to the speakers for optimum performance
Lower cost than buying a separate amp
Easier to set up and less calibration required
Fewer cables, plugs and connections needed
We also stock bi-amped monitors, too, which means that the woofer (bass speaker) and tweeter (treble speaker) each have their own amplifier. This helps to keep the output clean and tight - if you can afford to spend a little extra, this is a very worthwhile feature to look out for.
Within our range, most monitors accept standard Jack or XLR connections, but we even have systems that have a direct USB connection for even easier operation.
Overall, considering the importance of correctly-configured monitoring in the studio, powered monitors make sense because they enable you to simply plug in and get on with mixing, providing confidence that the entire monitoring system is doing it's job - don't forget your cables, though!