Early game - Or better known as laning phase. Everyone's focused on farming and ganking, trying to get an advantage early over the game. Can go anywhere from 00:00-20:00.
Mid game - First thing that comes to mind is the first teamfight. People will start grouping up and fighting or trying to catch off people for objectives. Normally an outer turret or two are down by then.
Late game - Champions are ~18 level with almost full build, people are going for inhibitors and Baron Nashor. Wards are really important around Baron as it's the most contested objective.