The fleas lifecycle is made up of 6 stages: These are:
Stage 1: EGG
Stages 2-4: LARVAL
Stage 5: PUPA
Stage 6: ADULT
This is important information when you're planning your flea treatment. If you just concentrate on removing the adult flea then you'll never cure the problem.
Of the 6 stages in the flea lifecycle only the last stage; the adult lives on your pet. Therefore when you treat your pet you're only removing the adults. Make sure you treat the environment in which your pet lives.