In what will mark the beginning of the end of Windows XP, Microsoft stopped selling that version of their operating system as of July 1.
There are some loopholes here, but the main point is that Microsoft is going to stop the main PC manufacturers (Dell, HP, etc) from selling new machines with XP on them, as soon as their current inventory of machines with XP is gone. This is not a very popular decision, especially given how slowly the Windows Vista operating system has been deployed.
Of course, if your computer already has it, this really doesn't affect YOU, only people buying new PCs. Also keep in mind that XP will likely be available on sites such as Ebay for a long time.
Of course, Microsoft's argument is that as they phase XP out, they can now afford to focus more on the Windows Vista user base, and therefore will be able to better support everyone.