Difference between WindowsXP and WindowsVista

The visual differences between XP and Vista are large in some areas, and small in others.
As for the technical part of it, it is the same thing; large differences in some areas, and small in others.
Overall, XP is more reliable, more user friendly, and has been around for a while so it has a large user base that can answer most questions about Windows. XP is also more compatible with printers, copiers, etc. XP also does not have User Account Control (UAC) which is a pain in the butt for Vista users. UAC asks you a question every time you do something, literally. For example, if you move something in the Start Menu, UAC will come up in a dialog box which takes you out of you work environment and says "Do you want to move this over there?" Basically, if you do something, Vista will ask you if you want to do it.
Vista on the other hand, has Direct X 10, which allows you to have more sophisticated graphics in games. Vista's desktop is graphically enhanced with 3D technology, which is basically the largest advantage that Vista has over XP. Vista has some minor advantages like faster loading of the Start Menu and quicker booting and shutdown.
If you must choose between the two, XP is the one to go with, for now. It is generally better, in all areas. If you want Vista for the looks, you should know better. Just like you should not judge a book by its cover
Windows XP was released in 2001. Windows Vista was released in 2007. Vista is new, and therefore is not as stable at this point. Windows XP has less features, but with the upcoming release of SP3, it will be very stable and secure.
Many people are not upgrading to Vista because of the performance impact of the new UI and the instability of it thus far. Wait until SP1 is released later this year if you are considering purchasing it.
As an authorized Microsoft Beta Tester for both products, I can honestly say that the difference is like putting lipstick on a pig. However I think the question you were really asking is should you buy Vista now or stick with XP. The answer goes as follows:
Wait for Vista Service PAC 109 to arrive before purchasing the original. Of course I'm being silly, but a thorough review of the history of the Microsoft OS products will prove my assertion to be true. Buy a US Savings Bond instead and wait it out.
Windows vista has lots of new stuff like aero view. It also has a new look. While Vista looks good it has very high system requirements and many of the older systems find it hard to deal with the high demand Vista requires of them, thus while it gives you a better look and infinitely more driver and options it does take a high performance system to get the best out of Vista. The biggest difference between Vista, XP and Linux is Vista's access and security, offering you a lot more options in securing your data and computer from other users. Visually Vista is also more user friendly displaying more information on the screen than ever before. Many users can access a single document from various system at the same time.