A large wizarding bookshop in Diagon Alley.
Flourish and Blotts is located "about halfway down Diagon Alley on the left hand side" (JKR/Twitter 8/9/15). Filled with shelves stacked to the ceiling, Flourish and Blotts is the primary - and perhaps only - supplier of textbooks for schoolbooks for Hogwarts (PS5), though of course that's far from all they sell. The shop holds occasional book-signings, such as when Gilderoy Lockhart stopped by in 1992 (CS4), and sells books via owl (HBP11) in addition to books purchased in-store. They also take advance orders for books that are in high demand, such as The Life and Lies of Albus Dumbledore (DH8).
In addition to the store's manager, who at times helps customers himself (PA4), the store also employs an assistant manager (paid 42 Galleons a month) (DP2) and an assistant (CS4). The job is not altogether easy, as wizarding books might attempt to bite or simply be completely invisible (PA4), and fights - like the scuffle between Arthur Weasley and Lucius Malfoy that sent books raining down on everyone's heads - are apparently commonplace enough that a classified posting for a new assistant manager requires that he or she be "good at breaking up fights" (DP2).
On Pottermore, Flourish and Blotts is immediately on the right, just inside the portal (Pm).