In the United States, and Western culture in general, there may be several bridesmaids for a bride, but the title of Maid of Honor is held is held by the bride's chief attendant, typically her sister or closest friend. The maid of honor at a wedding can be regarded as the guide for the bride, or bride's friend.
Maid of honor is at many times responsible for arranging the bride's dress and other preparations for the main wedding ceremony. In addition to that the maid is also incharge of carrying the groom's wedding ring to give to the bride during the ceremony, and other such customs.
Therefore, a speech given by the maid of honor usually have advice, wisdom and words of affection for the new couple that is getting married. Traditionally, this speech is given during the first half of the wedding.
Alternatively, this honor can be bestowed upon a man and would be refered to as "man of honor" or (in United Kingdom) "bridesman." The groom's equivalent is the "best man speech". A gender neutral phrase is "honor attendant".
Maid of honor and the best man are the key components of a sucessfully organized wedding. They themselves, take part in most of the organization with great excitement and honor.