Hi Matthew,
Sorry to hear this, and it's not good news. As to how/why it's happened, I can only guess one of two resaons. Either someone who knows nothing about SEO has decided s/he would "help" the whole SEO process and buy some links in. Or, and possibly more likely it may be a competitor who is doing a negative SEO campaign with the desire to lower your organic rankings, ensuring you will be penalised by Google.
You can now use the Google Webmaster tools and use the disavow tool to tell Google to ignore those links.
Hope that helps and all the best.
Gordon