They are after you from all over the globe. Looking for ways to exploit you. Ways to hurt you. Not political terrorists, but rather "info terrorists." Each month thousands of websites get hacked into and have hidden links inserted into the pages by people wanting their spam sites to rank highly in the search engines.
What they don't realize is that their targeted keyword phrases cause your site rankings to drop like a hot potato once the search engines are on to it. Google won't give you a warning when they see lots of links to these spammy sites. They will simply just stop sending traffic to the pages that contain the offending links. If the hacker has only added the code to a few of your pages the traffic drop can be quite small and it becomes almost impossible to diagnose the problem.
For detailed instructions on how to protect your site from hack attacks using the Google search engine, watch this video guide.