It doesn't mirror the content from your device, it actually streams it from "the cloud" like a Roku. Even when using Chrome, the webpages are sent directly from the cloud to the dongle.
One difference between Chromecast and a typical set top box like Roku is that there is no remote, everything is controlled from your phone, tablet or computer.
Once the content is selected and started streaming to the TV, you can use your phone, tablet or computer to do other things without effecting the streaming.
Hands On Review - Google Chromecast
