Diagon Alley

Diagon Alley (a pun on "diagonally") is a fictional street in London, and is effectively a magical High Street in J. K. Rowling's Harry Potter series. It is accessible to the wizarding world, to which it is something of an economic hub, but hidden from Muggles. However, Muggles are allowed access to it if they need to accompany their Muggle-born magical children. It is home to Ollivander's, makers of magic wands since 382 BC (likely the country's primary supplier), as well as the goblin-run Gringotts Bank and Madam Malkin's Robes For All Occasions. Other establishments include Flourish & Blotts bookstore, Eeylops' Owl Emporium, Quality Quidditch Supplies and The Leaky Cauldron, a famous wizarding pub and inn. For those shopping for Hogwarts equipment, Diagon Alley serves all needs.
There are other shopping areas besides Diagon Alley — the notorious Knockturn Alley is home to mainly stores of ill repute among many wizards (such as Borgin and Burkes, which sells Dark objects and cursed items); and the town of Hogsmeade.
One entrance to Diagon Alley can be reached on foot by passing through The Leaky Cauldron. The inn, invisible to Muggles, lies somewhere along the London thoroughfare Charing Cross Road. To reach Diagon Alley, one must walk up to a wall behind The Leaky Cauldron, and tap a brick, found by counting three up and two across, three times. Given the busy nature of the area, travelling to and from Diagon Alley is likely typically done by more magical means such as Apparition or by using the Floo Network.
The DVD of Harry Potter and the Chamber of Secrets includes a video "guided tour" of Diagon Alley, apparently shot on the original film sets.