Assembler is where you talk directly to the chip in your computer using its own instructions, without any higher order language 'getting in the way'. This means you can write very fast code, but it is much harder to read and therefore maintain.

Links

ASM

ARM Macro Assembler

AS

An ARM assembler

ExtBASICasm

Provides extra functionality to the BASIC assembler

HeyRick

A wealth of examples and explanations in all manner of assembler

soup kitchen

An online version of Matthew Bloch's ten part series he wrote on ARM assembler programming in Archimedes World