Heavy tanks as the name suggests are tanks that were heavier than their Medium counterparts, due to to increased armor thickness and bigger guns. Relatively slow, they were used to exploit openings in the enemy's line and create breakthroughs, while also supporting the infantry.
Heavy tanks are painfully slow and are very hard to maneuver. You should avoid taking them into urban areas as they can easily get stuck if they come under fire. It is best to have a Light Tank or a Lightly Armored vehicle supporting you, as you will be busy with dealing with enemy armor which leaves you open to being flanked by infantry. You shouldn't stay in one spot for a long period of time, as Heavy tanks have high profile and are easily spotted by infantry, making them a prime target for a bombing run.