Traditionally, a bride might have several bridesmaids, but only one 'maid of honour'. She would be the oldest and most 'senior' of the bridesmaids.

In the US, a maid of honor is an unmarried female who serves as the bride's chief attendant during the preparations for the wedding. She is usually at the bride's side assisting her with dress, veil, cosmetics, etc, while the bride dressing for her wedding. During the ceremony the maid of honor stands closest to the bride and often has a gown or corsage that differentiates her from the bridesmaids. (If the woman the bride chooses to fulfill the duties of the maid of honor happens to be married, she is referred to as "matron of honor.") The maid of honor usually holds the groom's ring for the bride during the ceremony. The maid or matron of honor is matched with the Best Man during the ceremony and at the reception. Bridesmaids are women (single or married) who the bride asks to participate closely in preparations for her wedding (they help plan her shower, help her address invitations, etc). In the US, it is customary that the bridesmaids wear matching dresses and shoes that they pay for themselves. (The dresses are usually chosen by the bride, but often she asks for input by the bridesmaids.) Here is an example - the maid of honor is the one at the far left holding the larger bouquet of flowers, and hers are peach and white, versus the smaller blue and white bouquets of the bridesmaids.