Windows XP was a hugely successful operating system and is still used by millions of people all over the world. Unfortunately, it's been in use for over many years, has been succeeded by three Microsoft operating systems (first Windows Vista, then Windows 7, and then Windows 8), and has entered a "retirement" of sorts.
When I talk of the "retirement" of Windows XP, I mean the end of support for the product from Microsoft. The self-service Microsoft Support site will still be available for Windows XP (probably forever) but there is no more free, or paid, support for the product.
In other words, no more security-based updates on Patch Tuesday, no more bug fixes, no more service packs. You're certainly welcome to continue to use Windows XP as long as you'd like but you'll probably find that newer software will be less and less likely to work properly. As many software makers have already done over the past several years, most or all will stop making sure that their product works with Windows XP.
See Microsoft's 365 Days Remaining Until XP End Of Support. The Countdown Begins article for all the technical details.
When Did Windows XP Support End?
Support (security patches, bug fixes, etc.) ended on April 8, 2014. Windows XP is no longer supported in Microsoft's monthly patches.
What Are My Options?
The easiest option is to do nothing. However, as I mentioned above, you run the risk of a less useable computer as time goes on. You also run the risk of being open to Windows security vulnerabilities that are discovered in the future.
The other option, assuming your computer hardware meets minimum requirements, is to upgrade to a newer version of Windows. Windows 7 or Windows 8 makes the most sense.