Madam Malkin’s Robes for All Occasions is a wizarding clothing shop located in Diagon Alley (PS5).
Though far from the only wizarding clothier around, Madam Malkin's seems to be the best, as it is the place where most Hogwarts students purchase their robes. Madam Malkin herself is a friendly, squat witch (PS5), though she seems a bit out of touch at times and got flustered when Draco Malfoy confronted Harry in her shop (HBP6).
Madam Malkin's is located next door to Flourish & Blotts, and in addition to selling Hogwarts school robes (PS5), they also sell robes which are "spangled, self-ironing, beautifying, slimming, fattening, lengthening," and "temperature-adjusting" (DP1) and they carry robes made by Whopperwear "for the outsize witch or wizard." A later advertisement states that every robe sold at the store is "self-ironing and repairing" as well (JKR).
Madam Malkin seems to have at least one employee; Harry sees another witch pinning up Draco's robes (PS6).
Twice, Hagrid accompanies Harry to Madam Malkin's, and both times he stops outside the shop and makes an excuse for not going inside (PS5, HBP6). The reasons he gives do not seem terribly strange, but it's an odd coincidence. -BB