Pinyin is the Mandarin word for "spelling", writing letters showing the sound of a word in place of a Chinese character. Today, it almost always has the sense of Hanyu Pinyin, the way of writing Mandarin sounds which has been used by the People's Republic of China from the middle of the 1950s. Other countries like the United States and Taiwan used other ways of spelling Chinese (such as the Wade-Giles way) for a long time but in the end started to use Hanyu Pinyin as well.
Pinyin uses Roman letters in a special way for certain Chinese sounds. For example, the pinyin letter C has a sound close to English /ks/ and does not sound at all like /k/ or /s/ by itself. Pinyin uses special marks to show the four sound qualities which are very important to using Mandarin clearly.