Pinyin is the phonetic romanisation of Chinese Mandarin. The Chinese alphabet consists of over 4000 characters, which can be pronounced in one of four tones. Pinyin Keyboard allows you to input Chinese characters on a Windows Mobile device, by typing in either Pinyin or English. The pop-up hints also translate Chinese characters back to their Pinyin pronunciation, with tone numbers, and their English equivalent... A very useful feature if you are learning Chinese.
Pinyin → Chinese → English
English → Chinese → Pinyin