Instead of relying solely on potions, which now have a cooldown, monsters will drop health globes that heal you and your party instantly during the battle. Globes heal a fixed percentage of health or arcane power. There are 2 sizes of globes: weak and medium. Boss fights each use globes in different ways, some will drop off smaller monsters, some will drop when the boss is damaged, and others will need to be found around the boss.
The main focus of Diablo 3 is combat. Every character is built for dealing large amounts of damage. There are currently eight types of damage that all have different attributes. To succeed in combat you need to customize your character wisely by picking passives and choosing 6 skills and modifying each of them with runestones. You can also bring one of three followers with you: Eirena the Enchantress, Kormac the Templar, and Lyndon the Scoundrel. When you aren't fighting you can craft new items by visiting one of the three artisans.
Every class has health and a class specific resource. Barbarians use fury, Demon Hunters have two: hatred and discipline, Monks are powered by spirit, Witch Doctors have mana, Wizards utilize arcane power.