"Said Ravenclaw, 'We'll teach those whose
Intelligence is surest.'"
-- Sorting Hat (OP11)
Rowena Ravenclaw was the founder of Ravenclaw House and one of the co-founders of Hogwarts, described as one of “the four greatest witches and wizards of the age” (along with the other three founders) (CS9).
Ravenclaw was "the most brilliant witch of her time" (JKR). She prized wit and intelligence in the students she selected for her house (OP11, FW), and had the motto "Wit beyond measure is man's greatest treasure" (DH29).
Rowena Ravenclaw had a fraught relationship with her daughter, Helena. Helena was jealous of her mother's renown and stole her prized diadem, which had powers to increase the wisdom of the wearer. Rowena was ashamed of this, and concealed it from everyone, even the other founders (DH31). When Rowena fell fatally ill, she attempted to get her daughter back for a reconciliation by sending the Bloody Baron after her, and Helena was killed as a result. Rowena Ravenclaw never recovered her diadem, and emotionally never recovered from her daughter's betrayal. While the facts became obscured by time, it became part of her legend that she died of a broken heart (JKR).
Other canon notes and references
She is said to have invented the ever-changing floorplan of Hogwarts Castle (FW).