Pressure type automatic pool cleaners usually have their own dedicated pump,but some run off of the existing return water flow. They usually have some type of bag which traps leaves and debris by the venturi action of water jets.
The first three shown below are different versions of the Polaris. This brand was the first pressure cleaner to incorporate a bag to trap debris-before that cleaners such as the Arneson Pool Sweep (the original) just kicked up debris through weighted jets at the end of long tentacle-like arms.
The Polaris comes with a back up valve that automatically backs the unit up every few minutes in case there is an area such as a step where the unit gets stuck.
One version of the Polaris cleaner requires no pump-but generally your pool has to be specially plumbed for this at the time of being built. If your pool was originally built with a suction port for vacuuming or a suction type automatic cleaner,this port can be re-plumbed to the return side to install a no-pump version of the Polaris.
The photos below are of the top of the line Polaris 3900 Sport showing the inside which has a stainless steel chain instead of belts. This is for people who want the best-a great looking cleaner that is designed to give many trouble free years of operation. About $1,000.00 retail not including pump,I may be able to get you a better price there is a link below the photos for more information on this cleaner:
The photos below are of the Hayward Viper and Hayward Phantom:
The above units are pretty similar except the Hayward Phantom has an extra feature:not only does it go along the bottom and sides of pool,but it comes to the top of the water like a surfacing submarine and cleans floating leaves on the surface then goes back to bottom in a programmed routine. Shown below is the extra large throat of these cleaners:
The cleaner below looks like a hummer and is based upon the Letro cleaner which in turn is a copy of the Polaris - the Letro originally had three wheels but was sued by Polaris and now has four wheels....Polaris is the superior cleaner between the two in my opinion. Under the photo is a link to troubleshooting these systems
The unit below works off of pressure (with white basket for leaves) OR suction (for dirt)