This is a browser take on the Nazi Zombies survival mode that Treyarch introduced in Call of Duty: World at War — a small map, an endless series of increasingly dangerous waves, and points earned by shooting that you spend on weapons mounted to the walls and on opening doors to new areas.
The loop is what makes it: staying in one room is safe early and fatal later, so you have to spend points expanding the map even though every new door is another way for the horde to reach you. It's a fan-built version rather than the official game, which remains a paid release.
No — it's a browser game built in the style of Treyarch's Nazi Zombies mode. The official Call of Duty games are paid releases on PC and console.
Train the horde. Run a wide circuit so the zombies string out behind you in a line, then turn and shoot down the line. Standing and fighting in a room only works for the first few waves.
Yes, with nothing to install — it's one of the heavier browser shooters, so close other tabs on older hardware.