Pinyin is the official stardard romanization used in mainland China to write Mandarin Chinese words in the Roman alphabet.
On a computer Hanyu Pinyin Input Method is only one of several available methods to type Chinese characters. It it based on Mandarin pronunciation and may be most popular among people who have leaned Mandariun Chinese as a foreign language. Native Chinese speakers may do much better with another input method. Especially if their Chinese is a dialect other than Mandarin, Hanyu Pinyin Input Method could be useless.