Hi Lithopsboy,
Canister filters were introduced by Eheim into Germany in the 1950's and into the USA in the 1960's. They are simply a type of power filter that sits on the floor and has 2 tubes running to the tank -- intake and out put. The big empty canister model allows the aquarist to use the filter as a triple threat filter -- biological, mechanical and chemical. In other words, you can use carbon, floss and a biological medium inside the canister. Now I use them as biological filters as I do not use carbon due to the number and amount of water changes I do. The amount of biological medium you can put in a canister allows for a big and effective biological filter bed for the tank. If you use Eheim canister filters, they come with all the biological medium and it lasts for years and years. It also comes with a carbon pad -- that I do not use and with a floss pad that I use and then when I need a new one (2-3 months) I just buy a bag of filter floss and make my own pad for a few cents instead of spending alot of dollars on the pre-cut pads Eheim has available. Regular filter floss works as well for me. This is where the real benefit of at least the Eheim canister filter comes in. They are a bit more costly than other filter types at first but over the long haul they save you lots of money as you don't have to buy expensive cartridges and ready made filter pads. The Eheim type of canister filter is silent to run. All you hear is the water entering the tank -- no motor noise. If you do use carbon in the canister and use a quality carbon like Marineland's, Canister filters make for extremely clear water. If you haven't noticed, Eheim is the brand of canister filter I like and use. Now, I use canister filters on my big tanks and old fashion plastic inside corner filters (very low tech) on my smaller tanks and they work great as little biological filters in a box. I fill them with Eheim biological medium over a small layer of filter floss on the bottom of the box and I have a great filter and one I can move around at will to establish new tanks right away. I keep extras in tanks bubbling away just so I have a cycled filter ready to go when I have to add a new fry tank or breeding tank.
Eheim cost varies. Check out the on line aquarium supply houses like That Fishy Place or Big Al's or Drs. Smith and Foster and see what they sell them for. They are ususally cheaper than stores -- but make sure EVERYTHING comes with the singe purchase. Sometimes places sell Eheims without the medium and quick disconnect values to lower costs and buying those things separately makes the cost higher than when you buy the whole kit as a unit!
I hope this helps. Eheim has a good site. I'd check it out.
Bobby