Dictionary Definition: 

“A compound of hydrogen and carbon, such as any of those which are the chief components of petroleum and natural gas.”

Selected from OxfordDictionaries.com

Functional Definition:

A hydrocarbon is a molecule comprised of just hydrogen and carbon including alkanes, alkenes, and alkynes. Hydrocarbons can be rings, branchs, or just saturated rows of carbon chains. Hydrocarbons are important because they store large quantities of energy in their bonds and they make up most of human made and used fuels. In plant ecosystems hydrocarbons are crucial in the water resistance of plant leaves and some bacteria can digest them as part of carbon cycling . Currently hydrocarbon byproducts of gas burning have many environmental scientists worried because they contribute to global warming.