Miu Miu is the secondary line from Prada. It was introduced in 1993 and is based on the nickname of the head designer, Miuccia Prada. It was also the first collection, at least, that was based on her wardrobe, things that she liked and based on pieces that she had.
It's a slightly younger, less expensive Prada line. Miu Miu offers ready-to-wear, leather goods, and shoes and it appeals to a slightly younger customer base. But really works some of the elements that are seen in Prada's main collection, you'll find variations of in Miu Miu.