Xbox – Difference between different ammunition

The one on the left is armor piercing shell (AP), the other one is high explosive (HE). The short story is, that AP shells have a better penetration, but HE shells do better damage (if it does penetrate) and might deal some damage even if it doesn't penetrate.

Usually you use AP shells, but switch to HE if the target tank can not be penetrated. However, it depends on the effectiveness of the shells on your particular gun.

On Xbox, you can see your tank and ammo stats by pushing "Y" while browsing your tank. This will reveal all penetration and damage values. (Tanks to Eric Tobias for this info!)

From the official wiki:

Armor Piercing (AP) Shell
This shell does the full damage listed on the gun specifications if it penetrates the enemy's armor. However, if the shell doesn't penetrate the target, it will bounce and do no damage whatsoever. It can also damage tank modules and crew if it hits at the right spot. AP shells are subject to normalization and lose some penetration value over distance.

High Explosive (HE) Shell
This shell has the highest damage value among all the shell types in the game, but also has a very low penetration value. These rounds have a very small splash damage area. It doesn't need to penetrate to damage the target, but the damage drops off quickly as the difference between the target's armor and the shell's penetration value increases. If it manages to penetrate the armor, it will explode inside the tank for the full damage listed in the specifications, plus HE deals additional damage to multiple internal modules and/or crew, depending on how high the HE damage value is. HE shells typically have a maximum damage value 25% higher than AP shells, rounded to the first significant figure.