Again I am mostly paraphrasing this response from the following website:
http://www.utm.edu/research/iep/o/ont-arg.htm
The ontological argument is specific to the case of our so called "being which no greater can be conceived". Read into Gaunilo's Criticism in which he tries to use the o. argument to prove that since we can imagine a greatest possible island it must therefore exist. The problem lies here: with what do we gauge the greatness of this island? Its abundance of fruit? There is no intrinsic maximum to which we can associate with fruit abundance--we can always imagine a greater abundance. In the case of a so called god we assign him properties/perfections such as omniscient, omnipotent, and omnibenevolent we can define in our own minds what these mean. Omnipotent, simple to understand, power over everything. If a being is unable to exert power in any situation it obviously does not hold the status of being omnipotent.
Secondly, it should be mentioned that St.Anselm developed to Ontological argument. The first uses the claim that existence is a property, that is it is greater to exist in the mind and reality than to just exist in the mind. Anselm himself realized a problem with this argument, which Kant would later explain, in that existence is not a property merely "a metaphysically necessary condition for the instantiation of any property". So his second version, the one quoted earlier by asaris, uses necessary existence as a property.
Nice to see such active participation on this thread, thanks!