Goodness! There is no way this question can be answered in any amount of detail on a simple web page. Dozens and dozens of books have been written about his legacy and accomplishments, so if you're interested in the difference he made, write a followup comment and I'll come up with a short list of 3 or 4 books to read. Start with the Book of Mormon though. 
 
Off the top of my head though, he facilitated the restoration of Christ's church after it had fallen into apostasy (with membership now around 15 million). With that came the restoration of the Holy Priesthood, which is the authority to act in God's name to do things like baptize. He was the means of translating additional ancient scripture into English, and his direction led to the LDS church members relocating to the Rocky Mountains (which in turn led to the eventual settlement of literally hundreds of cities in towns in the western US). 
 
He was just a man (as in, we don't worship him or anything), and his accomplishments were due to the fact that he was a tool in God's hands, but by any objective measurement he made an enormous difference - even if you don't believe in Mormonism at all!