According to smogon:
A Pokemon intended to take either physical or special attacks and hit back, but does not have to do so consistently over the entire course of a match.
Bronzong is a very potent tank. With Gyro Ball, Explosion, and massive defenses, it can hit enemies hard and take many hits effectively over the course of a match
A Pokemon intended to take either physical or special attacks extremely effectively and consistently over the course of a battle.
Skarmory's enormous physical Defense makes it one of the most sturdy physical wall in the game. It also has access to Spikes, Roost, and Whirlwind, three moves that Skarmory has many opportunities to use.
It would seem that these two differ in that a Tank can dish out a hit, where as a Wall plays as more of a spiker, only, that really describes a spiker, so I'm a little confused as to where a Wall sits in the metagame