In the world of Pathfinder, very little is based on the advancement of science. Sure, you'll get some cool technological weapons every now and then, but for the most part power stems from a) muscles, b) magic or c) a combination of the two.
The Alchemist, though? This is a guy who loves science - perhaps a bit too much - and employs it in battle on a regular basis. And, boy, does he make good use of it.
Alchemists are a little similar to magic users in that they are capable of creating effects similar to spells. Unlike Wizards, Sorcerers, Clerics and the rest, however, Alchemists do not employ magic: instead, they combine a series of ingredients, known as extracts, to generate effects similar to magic. It is science at work in battle, and they revel in the distinction.
That said, there are two things to note about most of the effects of extracts:
- First, most extracts are created to bolster the Alchemist's natural attributes.
- Second, unless you take a specific Discovery, Alchemists can only use their extracts on themselves. They become inert and useless if given to anyone else to use.
These, coupled with the Alchemist's discoveries - a series of revelations that grant the Alchemist some extra abilities - turns the Alchemist into, usually, one of two different kinds of characters: either a support character who can boost the stats of others and keep them healthy or, surprisingly, a potent physical force.
Why the latter? Two reasons. First, Alchemists are capable of creating Bombs daily that explode and have a range of useful, fairly awesome effects, as well as high amounts of damage. And, second, the Alchemist can use Mutagen to temporarily transform himself into a physically-powerful form with increased battle stats. An Alchemist using Mutagen - or the more advanced versions of the ability granted by Discoveries - and Bull's Strength and other such spells can be just as deadly, if not more so than, any Fighter or Barbarian.
As a consequence of this diversity, Alchemists can fill almost any role in a party, switching from one to another in an instant. That said, however, Alchemists may suffer a bit if they don't specialize in one region or another, allocating more to Strength and Constitution if they'll be fighting up front or more to Dexterity if staying in the back lines. In either case, pump a lot into Intelligence, as it's necessary to make the most of the Alchemist's Extract powers.
Whichever route you decide to go - whether you focus on support Extracts, Discoveries or even Bombs, which can offer quite a range of handy effects - make sure you try to be consistent. It's tough to create an Alchemist that balances all three aspects of the class, especially if your dice rolls for attributes are only so-so - or if your DM institutes a point pool. Look at what your party already has to offer before directing your Alchemist a certain way.