This article explains everything you need to know about sumps. Its geared for SW but most everything is the same, you just dont need things like skimmers for freshwater for example.
an added bonus is that they add water volume. hence if you have a 55g tank and have a 30g sump you really have an 85g tank. you can now stock the tank for 85g instead of 55g. that is considering territory and agro. of the fish kept. Many public aquariums do the same thing to allow more fish in the display, and 50% of the water kept in the background. It also allows you to hide equipt. in the sump tank so you don't see it in the display tank.
yeah. you can think of it as another tank hooked up to your show tank. maitinance can be done without affecting the main tank. and your overall water volume is more. you can also easily customize it to what you need and also use it as a holding tank
A sump/ trickle filter is a large container witch sits under your tank which if set up right will hold more water and give you a much larger good bateria surface area than any canister. It takes a bit to sort out and set up but once it is done it should be easyer to maintain than a canister eg-no having to disconect houses and carry out to be clean ect. it can hold your heater ,co2 setup and ease of placment of bags of carbin ect. They can be alot cheaper to put together than a good canister if you go diy. I am glad I put one on my 250L fresh water tank.