According to Wikipedia, Leonidas (Greek: Λεωνίδας - "Lion's son", "Lion-like") was a king of Sparta, the 17th of the Agiad line, one of the sons of King Anaxandridas II of Sparta, who was believed to be a descendant of Heracles. He succeeded his half-brother Cleomenes I, probably in 489 or 488 BC, and was married to Cleomenes' daughter, Gorgo. His name was raised to a heroic and legendary status as a result of the events in the Battle of Thermopylae.
In 480 the ephors sent Leonidas with the 300 men of an all-sire unit (soldiers who had sons to carry on their bloodline) and 6700 allies to hold the pass of Thermopylae against the hundreds of thousands of Persian soldiers who had invaded from the north of Greece under Xerxes. According to contemporary accounts Leonidas took only a small force comprised by his personal fighting unit, because Sparta's religious customs did not allow to send out an army at this time of the year. In addition, he was deliberately going to his doom: an oracle had foretold that Sparta could be saved only by the death of one of its kings, one of the lineage of Hercules. Instead it seems likely that the ephors supported the plan half-heartedly due to the festival of Carneia and their policy of concentrating the Greek forces at the Isthmus of Corinth.
Leonidas fell in the thickest of the fight, but the Spartans managed to retrieve his body and protect it until their final fall to enemy arrows. Herodotus says that Leonidas' head was cut off by Xerxes' order and his body crucified, due to his alleged hatred towards the Spartan King. This was considered sacrilege towards Leonidas, and unusual action on Xerxes' part. Immediately after he ordered the desecration of Leonidas' body, however, Xerxes felt remorse and, forty years later, Leonidas' corpse was returned to the Spartans.
He was buried with full honours, including a very un-Spartan display of wailing and mourning (Spartans normally accepted death in battle as a matter of course and disapproved of outward grieving, but the oracle at Delphi had ordered this along with the sacrifice of a Spartan king to preserve Sparta). A carved lion monument bearing the inscription below was dedicated at his death site commemorating the sacrifice of him and his men:
Go, tell the Spartans, stranger passing by,
That here, obedient to their laws, we lie. — ( Greek: Ώ ξειν', ἀγγέλειν Λακεδαιμονίοις ότι τήδε κείμεθα, τοις κοίνων ρήμασι πειθόμενοι) epitaph at Thermopylae (Simonides's epigram).
On a side note, two of the Spartans who were present at Thermopylae survived the conflict on the third day. One, Aristodemus, suffered an eye injury and was sent behind the lines and was taken back to Sparta with the retreating allies by order of the King. His companion, Eurytus, turned back despite having a similar infection which rendered him blind and died in the battle. Aristodemus redeemed himself through fighting with suicidal recklessness at the Battle of Plataea in 479 B.C., and returned home with his honor restored thereafter. A second Spartan, Pantites, was sent by Leonidas to attempt to raise support in Thessaly but returned to Thermopylae only after the battle's conclusion. Pantites hanged himself in disgrace after being shunned as a "trembler".