Ratfolk are humanoids with very ratlike traits... imagine Master Splinter from TMNT. They're small, agile, and very good at hiding, so they make extremely good Rogues and Ninjas. Their bonus to Dexterity also lends to ranged builds, and they have a racial Gunslinger archetype, so there is a lot you can do well with a ratfolk!
Ability Scores: +2 Dexterity, +2 Intelligence, -2 Strength. Ratfolk make extremely good Rogues, thanks to their small size and Dex bonus. They aren't particularly strong, though, so Strength-based melee builds are tough.
Size: Ratfolk are Small-sized, so they get a bonus to AC and Stealth, but at the cost of their speed.
Speed: Ratfolk have a base speed of only 20 feet.
Tinker: This ability gives ratfolk a bonus to some of the most useful skills in the game, Perception and Use Magic Device. The bonus to Craft (alchemy) can come in very handy for using poisons, as well.
Rodent Empathy: While you could build a character around making use of your +4 bonus to Handle Animal checks on rodents, I wouldn't recommend it. This is a nice skill boost for a Druid or Ranger, or any other character who takes an interest in mundane animals.
Swarming: This is one of the best (and possibly most broken) racial abilities in the whole game. Two ratfolk can share the same square with no penalties, and when they do so, they're