This is a quick overview of Toxic backlinks:
In general, a Toxic back link is any link that intends to manipulate the PageRank, or the search engine results.
It can be a link to your site or an outgoing link from your site, it will be considered a toxic if Google decides it wasn’t natural and is seen as an artificial attempt to boost rankings. Toxic links are also called “unnatural links” and can be considered a violation of the Google Guidelines.
from Andrew Brett Watson.com