There are actually 2 St. Valentines whose feast day it is today.

Seems that the apostrophe on St. Valentine's is written one letter too soon. While numerous early Christian martyrs were named Valentine, two are being celebrated by the Catholic Church today. One is Valentine of Rome, who was a Roman priest martyred around the year AD 269. His relics are displayed in a Church in Rome and Dublin.

The second is Valentine of Terni, who was the bishop of Interama around the year AD 197. He was martyred by Emperor Aurelian. Both Saints were buried on Via Flaminia, in Rome. Valentine of Terni's relics are in display at the Basilica of Saint Valentine in Terni. There is a third St. Valentine mentioned in early martyrologies but not much is known about him. 

Interestingly, neither Saints' histories mention any romantic elements in their biographies. The first romantic association with Valentine's Day didn't happen until the year 1382.

(Source)