The second law of thermodynamics outlines how energy can be changed from one form to another, but can not be created or destroyed. There are three categories: isolated, closed and open. The Earth is an example of an open system whereby energy and matter are exchanged. The universe is an example of a closed system. The Earth captures free energy, such as from the sun, and uses it for biological processes to replenish energy which is being degraded. This is like a a building project which starts showing signs of decay if regular maintenance work is not undertaken. Creationists mistake the Earth for a closed system, whereby there is no influx of energy from external sources.

Additional resources:
http://www.secondlaw.com/two.html
http://debunkcreationscience.hostse.com/thermo.htm
http://www.bcseweb.org.uk/index.php/Main/McIntoshsLaw