They are responsible for helping the bride (Try to keep her calm and not go turn into a bridezilla) Stand next to the bride and be a witness on the marriage license. Host a shower/bachelorette party. Help with the coordinate the other bridesmaids.
You are basically the brides maid/servant/go to person/boss of everyone else/puppy dog/carrier of everything she has/therapist/best friend/planner. You are the brides beotch. I just got done being one. Good luck. Get out of it if you can... :)