Say it with flowers! In the olden days, some suitors declared their affection through the language of flowers and not words. The young lover sent his beloved a bouquet of red roses, tulips, or carnations. If the lady sent her gentleman friend a bouquet of lilacs in response, she was communicating to him her shared affection. Isn't that romantic?
If she sent him ivy on the other hand, it was an invitation to be "just friends." That's a lovely way to communicate disinterest without hurting someone's feelings. I wish more people nowadays knew about this, instead of giving the "cold shoulder" treatment.
Today, most of the meanings behind various flowers are long forgotten or disregarded. We show up at the flower shop and pick the bouquet that looks the prettiest, without thinking about the message we're sending. You would benefit from a few tips about which flowers to buy for different occasions. You may even be able to have a conversation with someone based only on flowers!
Roses
Overall meaning: love. Roses are always a declaration of affection, even if it's just friendship. This is why red roses are so popular on anniversaries and Valentine's Day. Depending on the color, you can convey different meanings.

Red "“ passionate love
Pink "“ admiration/appreciation
Red & white "“ unity
White "“ purity
Orange "“ fascination
Yellow "“ friendship
Tulips
Overall meaning: declaration of love. Sending a bouquet of tulips is great option for dudes who are trying to figure out what flowers to send the woman they're interested in. They're not as flashy as roses, but have a deep symbolic meaning.

Pink "“ caring
Purple "“ royalty
Red "“ declaration of love
White "“ forgiveness
Yellow "“ hopelessly in love
Carnations
Overall meaning: pride and beauty. You might want to watch out for yellow and striped carnations if you're going to be sending these flowers, because they represent something you probably don't want to convey.

Pink "“ gratitude
Red "“ admiration
White "“ remembrance
Striped "“ refusal
Yellow "“ disappointment
Chrysanthemums
Overall meaning: fidelity, joy, and long life.

White "“ truth, loyalty
Red "“ love, sharing
Yellow "“ secret admirer
Lilies
Overall meaning: majesty, innocence, purity. The lily is always a delicate way to express these themes, and as such, are usually used as part of a bouquet with other flowers.

Calla "“ magnificence
Casablanca "“ celebration
Day "“ enthusiasm
Stargazer "“ ambition
Orchids
Overall meaning: exotic beauty. The orchid is one of the most delicate flowers because it requires stem reinforcements and a controlled environment. Send orchids to make a statement about the mystery of delicate, strong beauty.

Purple "“ royalty, respect
Pink "“ innocence, femininity, joy
Red "“ courage, strength, passion
White "“ elegance, beauty, humility
Yellow "“ new beginnings, joy, friendship
More Meanings
Lilac "“ First love
Daffodil "“ Chivalry, new beginnings
Daisy "“ Innocence, youth, motherhood
Hyacinth "“ Sincerity, constancy, playfulness
Queen Anne's Lace "“ Delicate femininity, sanctuary
Sunflower "“ Adoration, joy
Credit: About Flowers and The Flower Expert
Remember that someone giving you flowers might not know the meaning behind them, so tread carefully :)