Flash vs. Flash builder??

Hello,

I am trying to figure out which I should put most focus into. My  understanding which may be completely wrong is both can basically  accomplish the same thing using different methods...

If i want to create interactive interfaces which also contain  animations, which route would I go and does actionscript work the same  in each program?

I know the answer will vary based on who is answering, but if you knew  both programs what would cause you to choose one program over the other  for specific projects?

Thank you in advance for any and all replies