Maid cafés are a subcategory of cosplay restaurants found predominantly in Japan. In these cafés, waitresses dressed in maid costumes act as servants and treat customers as masters in a private home, rather than as café patrons. 
The maid costume varies from café to café but most are based upon the costume of French maids, often composed of a dress, a petticoat, an apron, a matching hair accessory (such as a frill or a bow), and stockings. Often, employees will also cosplay as anime characters. Sometimes, employees wear animal ears with their outfits to add more appeal.
Maid cafés were originally designed primarily to cater to the fantasies of male otaku, fans of anime, manga, and video games. They have been analogized as the otaku's equivalent of hostess bars. The image of the maid is one that has been popularized in many manga and anime series, as well as in gal games. Important to the otaku attraction to maid cafés is the Japanese concept of moe, which is a character trope in otaku media which describes young, innocent-looking female characters.

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-Wikipedia