A tank is a Pokemon who is inclined in the Attack and Defense of a certain Spectrum (Rhydon has very high Defense and Attack, so it's considered a "Physical" Tank, and a Pokemon like Gardevoir, with high Special Attack and Special Defense, are referred to as "Special Tanks").
A Wall is a Pokemon who can use their HP and Defenses to resist attacks. A Pokemon with high Special Defense stat, like Mantine, are considered "Special Walls." Pokemon like Aggron, with high Defense, are called "Physical Walls." Pokemon like Shuckle who have high Defense AND Special Defense, are called "Mixed Walls."