House Tyrell of Highgarden was one of the Great Houses of Westeros. It rules over the Reach, a vast, fertile, and heavily-populated region of southwestern Westeros, from their castle-seat, Highgarden. Formerly led by Lord Mace Tyrell, his mother, the indomitable Olenna Tyrell, stands as the last known member of the house. Mace's son Loras was a noted tournament knight (and, secretly, the lover of Lord Renly Baratheon). Mace's daughter Margaery married Renly when he crowned himself king, to cement the alliance between Renly and her father. Following Renly's death, Margaery was then married to King Joffrey Baratheon before his assassination at his wedding feast. She was then married to his younger brother, King Tommen Baratheon. Olenna Tyrell assumed House Tyrell's lordship as Margaery Tyrell, Loras Tyrell, and Mace Tyrell were killed by wildfire at the destruction of the Great Sept of Baelor. Under Lady Olenna, who has served as the head of the house since, House Tyrell has aligned itself with House Targaryen in opposition against Queen Cersei I for orchestrating the demise of most of the main branch of the Tyrell family.
The Tyrell sigil is a golden rose on a pale green field. Their words are, "Growing Strong".
(Source GOT wiki)