WordPress is a fantastic Open Source system powering over 40% of the World’s websites. This makes it a great target for hackers. A hack isn’t usually personal – bots are sent to probe websites looking for known security holes. On finding a security hole, a hacker will install code (usually automatically) to repurpose the website as a Spambot, or use it to host links to pornography sites, sell fake viagra, advertise knock off designer goods or some other sort of shady content.
Sometimes a hacker will install malicious code within your WordPress website, a bit like a virus – difficult to find and tricky to eradicate. Other times, the hacker will just delete your site and replace it with their own. They usually don’t have the good manners to make a backup for you first (are you backing up your site? See link further down for help if not).
Luckily WordPress’ mass adoption means that security patches are released multiple times a day. Installing these is simple, but you will need to take a bit of care.