What is the Ontological argument?

Immanuel Kant defined the Ontological argument as an a priori (knowledge that is independent of experience) argument that uses ontology to prove that God exists. It is an argument that is concerned with being. William Lane Craig defined the Ontological argument as one that seeks to deduce God's existence from His definition. 

An early form of the Ontological argument includes an example by St. Anselm of Canterbury, where he defines God as "that than which nothing greater can be conceived". The greatest thing we can conceive of must exist and therefore God exists.

Another form of the Ontological argument is presented by René Descartes in the "Fifth Meditation", where he defines God as a supremely perfect being. In order to be perfect, a being must possess the attribute of existence, thus we can prove that God exists by looking at his nature.