I don't know what it's called, but water spills through the grate into the 1st chamber where it get pulled down through various media (mechanical, biological, chemical), then forced back into the to the tank by a pump.
I'm strictly freshwater, so I never really saw filters like this until the all-in-one enclosed cubes started coming out several years ago.
My weekly maintenance is
1. turn off the pump,
2. siphon out water for water change
3. replace mucky filter pad with a clean dry one from the drawer
4. condition room temperature water, and add to tank
5. turn on pump again
6. go rinse out the pads and put them back into the drawer for next use