Flower girls are a decidedly Western tradition. Most scholars believe that the custom of incorporating young girls in wedding ceremonies began in Ancient Rome, though the modern idea of a girl strewing petals or carrying blossoms is almost decidedly English in origin.
The exact meaning of the tradition is a subject of some debate. To some the flower girl is meant to symbolize the bride as a child and to mark the passage of time and the transition from youth to maturity, marriage, and motherhood. In other places the flower girl is meant to invoke fertility. It’s common for the girl to lead the bridal procession with a basket of flower petals that she scatters along the aisle, creating a path for the bride and other members of the wedding party. Sometimes the variety of petal is chosen for its symbolic meaning, and the flower girl may also carry things like herbs or wheat, both of which have a more archaic traditional significance. The dress she wears while performing these duties is, by default, a flower girl dress.(from:wisegeek.com)