What is a pressurised pond filter and pump
Pressurised pond filters are ideal for small to medium size ponds, consisting of a canister which sits outside the pond and a pump within the water which pumps the water to it. From the filter canister the water is either returned direct to the pond or via a waterfall or other feature. The big advantage of pressure pond filters is that the actual filter canister can be buried so that very little is showing above the ground, making it much easier to hide than some other types of filter. Also the water is still under pressure when leaving the filter so you have a bit more flexibility when deciding where to position the filter. i.e. does not have to be above the water line of your pond.
Combination of pressurised and gravity return filter setup
These also make an ideal second filter to run alongside a gravity return filter but still only having one pump. Basically water would be pumped from the pond into the pressurised filter it then comes out of that along some more pipe to the gravity fed filter. This is the setup my pond ended up with, I first got the basic gravity return type filter box with pump and then after a while I add the pressurised filter. I actually then later upgraded the pump to a much faster one which was far too fast for the original filter so split the output from the presuraised filter so that only part of the water then went into the cheaper lower capacity filter. This setup might not be what you go for but shows that you can start of with a cheap basic filter and then add to it at a later date while still using the original filter.