Naresuan was king in Ayuthaya, Siam (now Thailand) from 1590 until his death in 1605. He earned the sobriquet "the Great"; others of Siam and Thailand's many rulers who are known as "the Great" Ramkhamhaeng, Narai, Taksin, Chakri, Chulalongkorn, Mongkut, and the present king Bhumibol Adulyadej. (This might seem like a long list, but it's not. Thailand's an ancient place and has had literally dozens of kings.)
Naresuan was born in 1555. His father was from an ancient family who traced descent back to the rulers of Sukhothai, while his mother was of Ayuthayan royalty. In 1569 Burma, Thailand's traditional enemy, ransacked Ayuthaya, looting the city and taking thousands of prisoners. They placed Naresuan's father on the throne and took young Naresuan with them as a hostage to ensure his father's compliance wtih Burmese rule. In 1571 his sister was presented to the Burmese king as a consort, and Naresuan was allowed to return home, though some stories say he went home for a visit and just didn't go back. In any case, back on Siamese territory his father sent him to the city of Phitsanulok to rule the area. It was customary to send sons and brothers out of the capital and into vassal principalities to run things and send periodic tribute to the centre of the region. At the same time Naresuan was named uparat, heir presumptive to the throne.
In Phitsanulok Naresuan showed his military capabilities in skirmishes with Cambodia, and is said to have led an attack on a Burmese stockade by scaling the walls with the blunt side of a sabre in his mouth. In 1581 his aging father delegated him to travel to Pegu in Burma to pay homage to the Burmese king. Thai legends say that while he was there he participated in an expedition against a Shan state and succeeded in taking a city after the Burmese crown prince had failed to do so; the Ayuthaya chronicles make much of the supposed animosity between the two princes.
Within a few years rumours began to spread that Burma was building a road to Ayuthaya, and that they planned to pillage the city and kill the king and his son. Naresuan moved with his armies into Ayuthaya, and from there managed to repel severaal advances between 1585 and 1587. In 1590 Thammaracha died and Naresuan became king. But the Burmese, and occasionally also the Cambodians, kept up their annual attacks on Ayuthaya. His brother Ekathotsarot was named uparat. The brothers had a chance to prove their courage in 1593, when the Burmese marched through Kanchanaburi, aiming to approach Ayuthaya from the west. The brothers, mounted on war elephant back, marched out to meet the Burmese attackers, and though the Burmese at first seemed to be winning and throwing the Siamese forces into disarray, the brothers held firm. When Naresuan spotted his old rival, the Burmese prince, he challenged him to a duel for the honour of their kingdoms, and quickly killed him. Ayuthaya was thus unified and freed from the Burmese yoke for a few generations at least, which is what makes Naresuan great in the eyes of the Thai.
Thereafter Naresuan set his sights on securing a strong place in the Asian milieu. He extended Ayuthaya's influence into Chiang Mai and other principalities in the north. He offered to send the Siamese navy to China to aid that kingdom in warding off Japanese attacks, though the Chinese declined. Wishing to expand his links to western powers, he signed a treaty with the Spaniards - Ayuthaya's only treaty with farang to that point was with the Portuguese - and initiated contacts towards the end of his reign with the Dutch.
Naresuan died in the north of Siam while leading an offensive against the Burmese.
Naresuan, incidentally, is credited with raising awareness of, and utilizing, muay thai (Thai boxing) for battle and for sport. The university in Phitsanulok is named after this great king.
Much of this information comes from the standard history of Thaland, Thailand: A Short History by David K. Wyatt.