Vacuum Cleaners

Hello.
I am interested in a new vacuum cleaner and have been looking at a lot of different models. The only question I have is that I see "Self Propelled" vacuum cleaners, what does that mean?

Self propelled vacuum cleaners are made to power the vacuum forward and backword making it easier to push. Although usually that added transmission can make the unit heavier than standard vacuums, causing it to be more difficult bringing up and down stairs.