According to Smogon, the definition of a wall id
A pokemon intended to take either physical or special attacks extremely effectively and efficiently over the course of battle.
Also according to Smogon, a tank is
A pokemon intended to take either physical or special attacks and hit back, but does not have to do so consistently over the course of battle.
In simpler terms, a wall or tank is a pokemon who is gifted in The areas of defense, whether it be in physical or special stats, and hits back.
Tanks and walls may also be thought of as counters to sweepers through typing and stats.
Notable sweepers can include:
The four listed above are prime examples of tanks/walls. Brongzong has well-rounded defenses, and high special attack to follow. Ferrothorn is also well-rounded defensively, and can pack a punch with strong physical attacks. Aggron is excellent in physical defenses and attacks, while Blissey is opposite: It is superior in terms of special defense and attack.
So yeah, walls/tanks are defensive pokemon who can retaliate.
I personally use them as damage sponges, especially annoying sweepers.