According to several reports both kernel.org and linux.org were hacked over the last few weeks. Showing that both linux isn't perfect and that users are the weakest link in any operating systems armour. In both cases nothing super secert from the users was stolen. The kernel.org attack is not an issue, as no one can update the kernel code or other software hosted at the site without a large nuber of contributors being told about the update. So with everything safe and the users passsword changed we can all breath a sigh of relief and walk away remembering that even simple things like password policies are important.
Bill Keys on LInuxSecurity.com does a really good job of both explaining what they are and some tips on how to avoid, remove, and detect them. It is a nice easy read that will help you get a good understanding of the topic. Check it out.