On the Raspberry Pi Downloads page, it describes it as
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.
Let's be blunt, simplicity like beauty is in the eye of the beholder.
The ArchLinux way describes it this way:
Simplicity is the ultimate sophistication. — Leonardo da Vinci
Simplicity is absolutely the principal objective behind Arch development. Many GNU/Linux distributions define themselves as "simple." However, simplicity itself has many definitions.
Arch Linux defines simplicity as without unnecessary additions, modifications, or complications, and provides a lightweight UNIX-like base structure that allows an individual user to shape the system according to their own needs. In short: an elegant, minimalist approach.