In Victorian times, certain flowers had specific meanings because the flower selection was limited and people used more symbols and gestures to communicate than words. But today, with so many flower choices, there are no rules - it's the sentiment that gives the gift its meaning. Many people assign their own personal meanings - a flower or color that might remind them of a special event or moment in their lives.
For those interested in the historic meanings of flowers, the Society of American Florists has compiled this list from a variety of different sources:
Copyright 2003 Society of American Florists