It's called refinishing, and it is discussed in detail on many TB threads if you search for "refinishing". Please search rather than ask for another rundown - it's pretty well covered.
There is excellent information online at
http://www.reranch.com as well, and that site and Stewart-McDonald.com are great sources for refinishing supplies.
IMO a good refin which includes stripping the existing finish and doing the work properly will take you 20-40 hours of labor. Doing plenty of your own homework before you start will enhance the quality of the finished project.