How were World War 1 and WW2 tanks used in war?

WW1: Tanks had one main purpose; to spearhead attacks on the enemy line so that friendly forces could pour through the breach. No proper anti-tank weapons existed in WW1, so they were almost invulnerable to enemy fire as they crossed trenches and wiped out enemy defenses. Unfortunately, the mobility of WW1 tanks was somewhat poor. They were slow, short on range, and constantly breaking down.

WW2: Tanks were used for the above-mentioned purpose but also in other ways. There were airborne tanks for paratroopers, amphibious tanks for marines, light tanks for various light armored duties, heavy tanks for assault, etc. In the early years of the war some countries tried to use light tanks as a substitute for cavalry, and in this role they failed miserably, as the enemy could hear them coming and be waiting to butcher them with anti-tank weapons. Armored cars proved much better for this purpose.