Microsoft will stop supporting Windows XP on April 8th, and is encouraging users to upgrade. If you're not sure whether or not you are using XP, you can visit the site amirunningxp.com which will tell you.
Amirunningxp.com tells you if you're using XP, and explains how long is left until the almost 13 year old operating system will no longer be supported by Microsoft. The site also helps users find out about upgrading to Windows 8, although for very old computers running XP, this is not much help.
Ending support for XP means Microsoft will no longer issue updates, security fixes, new drivers or software compatible with the operating system. The biggest issue for users is security - without Microsoft support, XP will become much more vulnerable to attacks and viruses. According to statistics on Wikipedia, as of January 2014, 28% of desktop PC still run XP, so a huge number of people will be affected by the end of official support.
To help users upgrade Microsoft released a free tool for transferring your data to a new operating system, PCmover Express which you can find out about here.