Check the GCC Inline Assembler HOWTO. GCC-Inline-Assembly-HOWTO and the Atmel AVR-8 instruction set.
That link uses the x86 instruction set, which is potentially rather confusing 
(Or at least it had me scratching my head at 2am....)
The AVR-specific equivalent, include a list of opcodes and constraints is at Inline Assembler Cookbook
Plus more links and some of my ramblings about getting started with inline assembler over at http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1207370768/2#2