As blogging has become popular, and wordpress has grown to be the most popular blogging software right now, that piece of software has also become a popular target for hackers. Why? Hackers may (ab)use the popularity of your blog to generate more traffic for other sites they run or manage. In short, they might add code to your wordpress blog that leads your visitors to their websites, and that’s probably not what you want.

A good post about possible malicious code is here: Did your wordpress site get hacked?