There is a really great article from theknot.com that explains what the duties of the maid of honor are (and also what the duties of the bridesmaids as well). Here is a link to it: just copy and paste in the address:
Basically it is the job of the maid of honor to take care of the bride. They are usually in charge of planning the bachelorette party, showers, etc, and attending all of the above. Make sure she isn't told of any problems on her day (anyone with problems comes to you or someone you/bride delegates), you are there to toast her, keep her sane, hold anything she needs held (flowers during the ceremony, grooms ring, etc),and keep her on schedule before the ceremony. There are a million small things a maid of honor can do to help her friend on her special day.
If for some reason that link above doesn't work, just go to www.theknot.com and type in "maid of honor" under search. You should be able to find different articles on the subject - one of them being "MAID OF HONOR - YOUR DUTIES IN DETAIL"