Followers can be real lifesavers.
As you progress through the game, you'll be joined by three followers:
Only one follower can fight alongside you at a time (other allies, like pets or major NPCs, can join you simultaneously).
Recruit your followers from camp.
Followers fight for you automatically, but you can choose skills for them that affect their behavior in combat. For example, if you choose the Charge skill for the Templar, he’ll begin fights by rushing at enemies with a stunning attack; the Heal skill means he will restore your Life during the course of a battle.
Followers who aren’t currently traveling with you remain in your camp. To recruit a follower to join you (this will replace your existing follower if you have one), walk to their location, interact with them, and select ‘Hire’.
You can also interact with your followers to talk with them and learn about their history or ask for their advice on your current quests.
As you grow more powerful, your Followers will gain experience and unlock new skills. You can also gird them with improved equipment, though they can only equip accessories or appropriate hand slot items and weapons.
Upgrade your followers' gear and skills whenever you get the chance.
To upgrade your followers, access their inventory page.
You’ll see a list of two available skills (two per category). You can highlight any skill to learn about it before choosing the one you want, and you can retrain your followers’ skills here as well. This window will also allow you to adjust their equipment, swapping it out for anything in your inventory that they can use. You can equip Followers with a total of 14 different item slots which include the Follower Relic Item, Helm, Shoulders, Chest, Gloves, Bracers, Belt, Two Rings, Neck, Pants, Boots, Main-Hand Weapon, and Off-Hand. When equipped on followers, specific equipment can Emanate powers. Items that Emanate allow you to gain the intended effect of certain Legendary Powers and Set Bonuses when equipped on the hired Follower. To equip your follower, click on the item you’d like them to wield, and drag it to the appropriate slot in their inventory.
Choose the follower and skills that best complement your play style. If you like to fight on the front lines and soak up damage, pick a follower that will heal you or delay your enemies’ attacks. If you need extra killing power, load your followers up with skills that allow them to deal damage over time or attack multiple targets.
Source: Diablo III