From the Raspberry Pi downloads page:
If you’re just starting out, this is the image we recommend you use. It’s a reference root filesystem from Gray and Dom, containing LXDE, Midori, development tools and example source code for multimedia functions.
Arch Linux ARM
Arch Linux ARM is based on Arch Linux, which aims for simplicity and full control to the end user. Note that this distribution may not be suitable for beginners.
But this isn't much to go on, so what are the benefits of using Arch Linux on the Raspberry Pi over the standard Debian build from the Raspberry Pi foundation?