Armour material comes in many varieties. From the basic steel armour, to composites, explosive reactive armour, and non-explosive/energetic reactive armours (NERA).
Armour material has much to say about the threats that an AFV protects its crew from. Back in the 1960s and '70s, HEAT warheads were a major threat to any armoured vehicle, since no armour technology had yet been developed that could effectively defeat HEAT projectiles. Tank protection in the '60s was primarily rolled homogenous armours (RHA) or cast steel, with some APCs like the M113 using aluminum. The solid cast steels used in the '60s and '70s provided poor protection against HEAT warheads, as making them fully protected would make the vehicle prohibitively heavy. Tanks of the time were generally protected against medium caliber sold shot and APDS rounds frontally, but not much else. During the 1973 Yom Kippur war, AT-3 Sagger (rus: 9K11 Malyutka) missiles used by Egyptian forces caused horrible tank losses for the Israelis. Explosive reactive armour, and use of ceramics in composite armour arrays, was a major breakthrough in armour technology. Chobham armour, consisting of ceramic tiles in a honeycomb configuration, sandwiched between steel plates, has a very high RHA equivalent (RHAe) against HEAT projectiles, usually about twice that of RHA steel and only a little less RHAe against AP than RHA steel.