Ok so now we know it is due to the Penguin update.
The first step to do will be analyzing every single on of your backlink. Use Webmaster Tools and export all the backlinks that are pointing to your website. Next you will have to check out each single backlinks to see the quality of the link. I like to use the Mozbar addon to quickly check what the MozRank and MozTrust of the page is where the backlinks to your site is located.
Once you got the full list of links you want to have gone, you will first have to manually try and remove the links. This could be relatively easy if the link for example is from a directory where you had to make a profile first in order to create a listing. In that case, you can just delete the listing. Otherwise you will have to email the owner of the website if they could remove your backlink.
Now if, let's say 2-3 weeks have passed and you still haven't heard back of some the webmasters (even though you contacted them), you can go ahead and use the link disavow tool. This tool however has to be used with high care.
Have a read of http://searchengineland.com/google-launches-disavow-links-tool-136826 for an understanding of the tool.
If you don't fully understand the tool, please come back to the SEOMoz forums and ask for help, you need to fully understand how it works.
Good luck and feel free to ask any other questions!