You need to decide which computer and browser is where you will make initial changes to your bookmarks. Usually this will be the browser that you use most often on your main computer. From here, you can sync across your other browsers and any other computers you have. Xmarks allows you to merge bookmarks on Xmarks server with your browser, keep bookmarks on your browser discarding those on the server (main bookmark), keep the bookmarks on the server while discarding those on your browser. You also can integrate passwords (not recommended), and open tabs as well as enabling automatic synchronization. This is a great timesaver that eliminates the need to do this manually. A feature of Xmarks is also that you can be away from your computers but can retrieve that necessary site that you wish to access but cannot remember the name by accessing the website and logging into it. I found the instructions easy to follow and the program is user friendly.
A paid version of Xmarks ($12 annually) allows you to sync between your smartphone and track and backup all changes to your bookmarks. My personal opinion is that this is not needed.
While there are ways to save your bookmarks on your computer and sync using your saved bookmarks on your computer, Xmarks is a free way to do this saving time and aggravation.
I also want to extend my best wishes to those celebrating a very Merry Christmas and to all a very Happy and prosperous 2011.