what is a carbon sink?

A carbon dioxide (CO2) sink is a carbon reservoir that is increasing in size, and is the opposite of a carbon dioxide "source". The main natural sinks are (1) the oceans and (2) plants and other organisms that use photosynthesis to remove carbon from the atmosphere by incorporating it into biomass and release oxygen into the atmosphere.

Basically it's anything that will collect carbon dioxide.

A bunch of crap spouted by greenies.

By definition, a carbon sink is anything that contains carbon, except carbon dioxide that is not dissolved in another substance. So you are a carbon sink. Trees, grass, oil, natural gas, coal, the earth, the oceans.

However, more often the term is used to describe anything that takes carbon dioxide out of the air.

This would make sinks anything that absorbs CO2 and converts it to something else or stores it so it cannot be re-released to the atmosphere. The ocean doesn't count but all the organisms that convert CO2 to O2 inside it would.

Oil is a great carbon sink and no doubt thats why nature uses this method. Another one is calcite and dolomite rocks also used by nature for storage of carbon. In fact most of the carbon on Earth is stored in rock formations.

Anything that is capable of sequestering carbon from the environment. One LITERAL carbon sink is the ocean. Organic 'snow' falls to the deep oceans, where it stays until it's subducted into the earth's mantle.

Every plant that makes it food with sun and CO2 and water. The plants give us back the O2 and keeps the C to make its food.