There are a couple of St. Valentines, or at least two different ways the story goes.

Version one makes Valentine to be an early Christian bishop who secretly married fellow Christians during Roman rule.  He was eventually caught and then martyred.

Version two makes Valentine an imprisoned man who fell in love with a jailer's daughter.  On the day he was put to death, February 14th, he left a note for the jailer's daughter that said, "I will always love you, Your Valentine." 

At least that's the version I got yesterday from a teacher, I haven't looked anything up on Wikipedia yet.