Prince Joffrey Baratheon is known to the Seven Kingdoms as the eldest son and heir of King Robert Baratheon and Queen Cersei Lannister. A member of House Baratheon of King's Landing, his siblings are Princess Myrcella and Prince Tommen. After his defeat in the War of the Five Kings, he and his brother are named as abominations of incest by the faith and the crown. He was once betrothed to Sansa Stark.
Joffrey grew up as a spoiled and indulgent child with a cruel streak within him. His father, King Robert, is deeply disappointed with his son and feels little affection for Joffrey. When it is revealed by Eddard Stark that he is not Robert's trueborn son and Stannis Baratheon is the rightful heir to the Iron Throne, he had Lord Stark executed on the grand sept of Baelor.
Joffrey's rule is marked by inadequacy, and only his uncle and grandfather are able to maintain what it is for his short rule. His reign is marked by cruelty and the decreased quality of life in King's Landing, making him an unpopular king among the smallfolk. His rule is put to an end when the forces of Renly Baratheon commence the Siege of King's Landing. The city is starved and eventually the gates are opened by the angry and starving clansmen of the Vale, leading to a victory for Renly Baratheon.
Joffrey is killed in this conflict, as was his grandfather, Tywin Lannister in his attempt to relive the siege. His mother is executed for crimes against the Crown and his uncle, Tyrion Lannister and his younger siblings are exiled to Essos where they are never heard from again.
[Link to Baratheon FT]