What is Chillingo.ltd? What Do They Do?

There are a ton of (mostly) highly rated games in the Apple appstore under the company name 'Chillingo.ltd', but the games are so wildly different from one another, that I can't understand how they could develop the hundreds of them internally.

What's the deal with this? Is 'Chillingo' just a pseudonym for a group of developers?

It's a publishing company.

From their website:

Chillingo is a leading Independent Games Publisher and a division of Electronic Arts. The company publishes innovative games and software for various platforms.

They don't develop games. Developers come to them and make various arrangements for their games to be published/distributed. It works in a similar way to how games for PC and consoles are produced: for example, Deus Ex: Human Revolution is developed by Square Enix, but published by Eidos Interactive.

Game publishers perform a variety of functions. They are generally responsible for marketing and production costs which might include the development of the games they publish.

Chillingo is simply a publisher for games on the App Store. Developers submit their apps to Chillingo so they can publish it instead of having to do it themselves. Chillingo handles the marketing and other things like that, so the developers don't need to worry about it. Also, Chillingo games are generally very popular, so a Chillingo published game will likely get a smaller development studio more sales and a much higher exposure level on the App Store than if they were to publish it themselves.