What is it?

It could be explained pages about, but to do it it is already Wikipedia, so basically:
It is a fossil fuel formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock (which were sediments at first) and undergo intense heat and pressure.

When tiny organisms die, they sink to the bottom of the sea and are mixed with mud and silt. Over time, hundreds of feet of mud containing the organisms accumulate. Bacteria removes most of the oxygen, nitrogen, phosphorus, and sulfur, leaving mainly hydrogen and carbon. Lack of oxygen keeps the animals and plants from decaying completely. The partially decomposed organisms create a slimy mass, which is then covered with layers of sediments. Over millions of years, many layers of sediment pile on top of the once-living organisms. The weight of the sediment compresses the mud into a fraction of its original thickness. When the depth of burial reaches about 10,000 feet, heat, time and pressure turn the organisms into different types of petroleum.

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