The ref given by kveeti is good, basic and clear.
Some things not mentioned or stressed is that these are closed systems other that the intake and return, so can operate with efficient pumps for minimal operating costs. They are more expensive that HOB type filters by a significant margin, but they tend to have several times the media volume of such units (another efficiency point) and if well designed and operated, do not allow significant bypass of the filter material. Good brands should last indefinitely - I have multiple canisters >20 years old, a couple >30. Customization of the media was mentioned. Operator control over placement and control of the return water flow are pluses as well. The best are totally silent in operation. Downsides include initial cost of the unit and the media, older models were not as user friendy on maintenance as HOBs, and the wider selection of media and positioning requires more thought in selection and setup.