The Lannisters are an ancient house, descended from Andal adventurers and claiming descent through the female line from Lann the Clever, a legendary trickster of the Age of Heroes. They ruled as Kings of the Rock until the Field of Fire, where King Mern of the Reach died. King Tommen II had lost his life and the family’s Valyrian steel greatsword Brightroar when he had gone on his fool’s quest to Valyria (Gerion Lannister, Lord Tywin’s youngest brother, had gone in search of it and had never returned as well). King Loren of the Rock managed to survive and bend the knee to Aegon. The westerlands are rich in gold, making the Lannisters the richest house in the Seven Kingdoms. Their motto is, “Hear Me Roar!”
Lord Tywin Lannister was King Aerys’s Hand for twenty years but was replaced with a string of unlucky men. It was he who led the Sack of King’s Landing, at the end of the civil war. His daughter Cersei he wed to King Robert. His son and heir is the Kingslayer, Ser Jaime Lannister, twin to Cersei. His youngest son, Tyrion, is known as the Imp for his misshapen, dwarfish body and his cynical wit. His only remaining brother, Ser Kevan, is his closest advisor.
After King Robert’s death, caused by Cersei’s purposeful endangerment of his life, his son Joffrey (in reality the incestual product of Cersei’s long-running affair with her brother Jaime) was crowned even as Lord Eddard Stark was imprisoned before he could reveal the truth to the Seven Kingdoms. Lord Eddard was executed, beginning the civil war in full. The Lannisters have suffered defeats at the hands of young Robb Stark, with Ser Jaime (now Lord Commander of the Kingsguard) captured and imprisoned at Riverrun.
Tyrion served as King Joffrey’s Hand for some months until he was wounded in battle and replaced as Hand by his father, who had successfully lead the combined Tyrell-Lannister hosts against King Stannis Baratheon. Ending his betrothal to Sansa Stark, held as a ward of the crown, King Joffrey has agreed to marry Margaery Tyrell. In the aftermath, Tyrion was made Master of Coin and was married to Sansa Stark.
Ser Jaime was freed from Riverrun by Catelyn Stark, which led to Rickard Karstark murdering two Lannister youths then held captive there. In the course of his journey in the company of Brienne of Tarth, Ser Jaime lost his hand to the Brave Companions and determined to now try to be a better knight. He refused to give up his place as Lord Commander of the Kingsguard and has since had a falling out with his sister. And his brother as well, for Tyrion was to be executed for murdering King Joffrey (he did not). Jaime freed Tyrion but also admitted to him that Tysha was never a whore, that it was a lie Lord Tywin forced on him. Tyrion went on to murder Lord Tywin and his mistress Shae before departing the city.