Follower

A follower is a type of NPC often encountered in the dungeons. Followers are generally part of quests of various complexity, and are somewhat akin to smarter, more clingy versions of the wandering NPC Barbarians found in Act 5 of Diablo II.

They are not meant as replacements[1] for true mercenaries; you do not equip them or gain benefits from their special skills, you can not heal them directly, and they do not accompany your character through numerous battles and travel with you between towns or Acts.

There may be true mercenaries in Diablo III, akin to the mercenaries hired, equipped, and resurrested in Diablo II. The D3 Team has not yet committed to this feature, though.

Mercenaries in Diablo III

Nothing is yet known about true mercenaries in Diablo III. The team has said they hope to include full mercenaries as a feature, and that if they do they'll be bigger and better than mercs were in D2. But they've given no details yet. [2]

Do those NPC's provide abilities to the hero, like a proper party system?

Julian Love: That's also not yet finalised. We're trying to make them definitely a step above the henchmen of Diablo II, but how big of a step is something I can't say yet. Hopefully next time we reveal something I can show you more.

The D3 developers say things like this about features that are either: 1) not going to be in the game at all, 2) not going to be in the game until an expansion, or 3) going to be in the game but are being kept secret until they're ready for a full reveal.

The developers answered questions about topics such as the PvP Arena, crafting, NPC merchants, and other topics in this fashion, and all of those aspects of the game were eventually revealed in full detail as big upgrades over how they were handled in Diablo II. In light of that, it's not unrealistic to hope or even expect that Diablo III mercenaries will be a full game feature and as good or better than they were in Diablo II. If not in the initial game, then certainly in one of the expansions.

Known NPC Followers

Players have found various NPC followers in the playable demos at Blizzcon in 2008, 2009, and 2010. Followers are usually part of various scripted events and quests. In the known examples players found NPC warriors who needed assistance fighting off large groups of monsters, explorers who needed help reaching a special treasure, injured humans who requested escorts, and even an NPC merchant who felt his honor had been insulted by another NPC, and offered to sell his items at a discount if his reputation was restored.

With so many NPC follower events and mini-quests in the three playable demos so far, it seems very likely we'll see quite a number of these adventures and events in the final game.

A few examples:

The Barbarian finds Deckard Cain with three NPC archers in the dungeon in the WWI gameplay movie. The Barbarian orders Cain to follow him to safety, and the Archers come along to assist. They join in the battle and do their best, but are picked off one by one until only the Barbarian and Cain reach the end of the level.

The Escort Mission was a mini-quest in the Blizzcon 2008 demo. Players found an NPC guard captain who asked for assistance in locating the rest of his soldiers, as they'd become separated in battle. The Captain accompanied you as you explored the surrounding areas, and as each of his lost soldiers were found, they joined in as well, acting as durable, but low-damage, tanks.

Other such events were part of the 2009 and 2010 demo as well.

NPCs assist the Barbarian to rescue Cain.