tank vs bruiser
What is expected of them. A tank absorbs and controls, its damage is secondary; a bruiser absorbs and hits, it has to kill to be useful. Both stand in front, but not for the same reason.
tank
The tank is the champion who holds the frontline: lots of health, resistances, crowd control to lock the enemy down. It engages, protects, absorbs. Its damage is low, and that is fine. "we need tank" asks for someone willing not to get kills.
open the page →bruiser
The bruiser, or fighter, takes fewer hits than a tank but hits much harder. It wins duels, holds a side lane, and threatens enemy carries up close. It needs gold to matter, where a tank works even when behind. "we need a bruiser top" asks for someone who can win alone.
open the page →Why they get mixed up
Both take hits in front, hence "frontline" for either. The mix-up is costly in champion select: a team with two bruisers and no tank has nobody to engage or protect, and a team of tanks kills nobody. Asking "who engages, who kills" settles the pick.