I say "legend" because there are unfortunately no hard records of his life, as opposed to the case of some other popular saints; there's only sort of a church tradition that remembers him. Historians disagree on which specifics are actually truthful, but I'll try to compile all the stories into one....
Saint Valentine lived in the 3rd Century A.D in the Roman Empire, which was then pagan. He was a Christian priest who had two clandestine ministries: one was ministering to the physical and spiritual needs of imprisoned Christians as they awaited their martyrdom. The other was performing marriages (see, Emperor Claudius II had banned marriage b/c he wanted more single men available for his army).
The emperor, intrigued by him, had Valentine brought before him. Valentine, instead of begging for mercy, preached the gospel to him! The emperor was so impressed with his boldness that he offered to spare his life if he would just renounce Christ and bow to the Roman gods. Valentine said something along the lines of, "Your gods are but the invention of wicked and vile men." Well that pretty much sealed his fate. He was thrown into prison to await execution.
While in prison, he befriended his pagan jailer and his blind daughter and prayed for her constantly. God restored her sight, and the jailer and his whole family became Christians (and eventually met their own violent martyrdom). On the eve of his execution, he wrote a letter to the daughter of his jailer in an attempt to comfort her. He signed it, "From your Valentine."
He was killed on February 14, 269.
Well, February 14 had also been a big holiday in the Roman empire, kicking off the Feast of Lupercalius. Basically the teenage boys would pick the names of girls out of a lottery and whomever they picked would be their sexual partner for the duration of the festival. When Rome became Christian, the Pope decided this feast had to be amended. So they started picking Saints' names out of a lottery and devoting themselves to that particular saint for a period of time. Because Valentine had been killed on this day, they decided to name the holiday for him.
More recently, the Catholic church decided that since they couldn't be sure of the specifics of his story, that maybe they shouldn't include Valentine's Day in the official church calendar.
If anyone has anything to add to the legend please do.
I like to remember this story; it lends so much more meaning to today than just buying a Hallmark card and some flowers or chocolate. Valentine showed sacrificial love in risking death just to show kindness to those already condemned; he loved and prayed for the daughter of his own jailer; he thought only of her angst on the night of his own execution; he acted illegally and dangerously on his belief in the sanctity of marriage; and he offered even Claudius the Cruel a chance to receive salvation.