Debian ARM Port
Arm is RISC Architecture originally created for the Acorn Archimedes and subsequent desktop machines. These days Arm targets low-powered embedded systems, such as phones and routers and PDAs and is the most popular CPU architecture on the planet. Systems capable of running Debian/Arm are cheap and easily available, but usually with limited ram and IO, which makes finding buildd-suitable machines hard.
ARM ABI variants
Debian ARM was traditionally been compiled to the 'old' arm-linux ABI (specifically arm-linux-gnu) and for the arm v3 instruction set, allowing it to run on machines as old as the Acorn Risc PC and Psion 5. This is designated by the suffix 'arm' in Debian packaging. The last release of Debian including the "arm" port was Lenny (5.0.x).
A new 'EABI' variant uses the newer ABI. This is designated 'armel' in Debian packaging, and has been supported from lenny onwards. Yet another new variant (armhf) is currently in preparation and will hopefully ship with squeeze. See ArmPorts for more links and an overview.
There used to be much more information here about the armel port, including supported hardware. That has now moved to ArmEabiPort instead.