"top" output

Hello list, I have been trying to figure out what does the cpu usage output from top mean. I get: Cpu(s): 24.8% us, 11.3% sy, 0.0% ni, 55.0% id, 0.0% wa, 0.0% hi, 8.9% si but I cannot figure out what does "wa", "hi", and "si" stand for. Can anybody help? Thanks. -m