Arm is a 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.
ABI variants
Within the official Debian archive there have been four different ports to various flavours of the Arm architecture:
arm64: Arm64Port - the latest port, for the 64-bit Armv8 architecture. First released with 8 (jessie). GNU Triplet: aarch64-linux-gnu
armhf: ArmHardFloatPort - the latest 32-bit port, using the hard-float version of the "new" ABI (EABI), targeting Arm v7 and up. First released with 7 (wheezy). GNU Triplet: arm-linux-gnueabihf
armel: ArmEabiPort - newer port using the "new" ABI (EABI), supported on Arm v4t and higher. First released with 5.0 (lenny). This port is also known as armel. GNU Triplet: arm-linux-gnueabi
OABI: the "old" and obsolete ABI. First released with 2.2 (Potato), last released with 5.0 (lenny). GNU Triplet: arm-linux-gnu
There are or have-been at least two unofficial ports outside the Debian infrastructure.
Raspbian - Uses the hard-float version of the "new" ABI (EABI) like Debian armhf but targets v6 rather than v7. Currently tracks Debian jessie and stretch. Primarily but not exclusively targeted at the Raspberry Pi. GNU Triplet: arm-linux-gnueabihf
- armeb - Big-endian OABI port targeting the linksys NSLU2 and similar. Interest fell after a method was determined for running little ending Linux systems on the NSLU2. Active during the sarge timeframe and now abandoned.
Arm64ilp32Port - abandoned port, for the 64-bit armv8, with the ILP32 ABI instead of the default LP64 ABI one