cannister filters are external filters with inlet and outlet tubes that run to and from the tank. these are usually rather efficient due to the way the water is passed through several types of media.
i assume you mean undergravel filter. these are filter plates which sit under the gravel, with an uplift tube for the water to be forced up (either by air stone or powerhead) which in turn sucks water through the gravel to the plate, using the gravel as filter media. these are outdated now and not very efficient.
