Outline the key features of the ontological argument

The aim of this question is to simply outline the main features of the ontological argument. This requires very little analysis. Rather, the point is to clearly and concisely explain the features of the ontological argument that differentiate it from other arguments for the existence of God.

The ontological argument was initially developed by St Anselm, so this is a good place to start. The first paragraph should outline Anselm's argument. Anselm was later criticised by Gaunilo and developed his argument in reaction to this. Both Anselm's original and revised arguments are important in understanding the ontological argument.

There are several other philosophers who you might want to mention when outlining the key features of the ontological argument (Kant and Descartes are important examples). Once you have outlined what the argument actually argues, it might be a good idea to acknowledge that the real distinguishing feature of the ontological argument is that it is a priori rather than a posteriori.