Valentine is the name of several saints who are commemorated on February 14th every year. In the last 150 years, the custom has grown of sending cards or messages to the one that you love. The cards are called Valentines and the loved one is also the valentine. This custom began in the USA and the first commercial cards were produced in the 1840s.
It is now a common custom in many other parts of the world. In mediaeval times, it was believed that the birds chose their mates on Feb 14th and so that connection with love arose. Choosing a loved one on this day might also have some connection with the Roman festival of Lupercalia, which used to take place at this time of year.
St Valentine himself was a Roman martyr who lived in the 3rd century. He is now the patron saint of lovers, but he did nothing in his lifetime to warrant this.