Tar sands, also called oil sands, are a type of fossil fuel deposit, which has only fairly recently been considered as feasible sources of oil.
The extraction of tar sands is a matter of concern due to the large carbon footprint and the local environmental damage of the process.
Tar sands are the "high hanging fruit" - requiring a lot of energy to extract and purify. Advances in technology have gradually allowed more extraction to be energetically profitable (i.e. decreased the amount of energy required to extract tar sands).