Understanding the Ontological argument for God's existence

Defining God as the greatest possible being
Greatest possible being is the being which has good characters to the greatest possible extent. Greatest possible being is loving , merciful , truthful , compassionate , omnipresence , omnipotence and among other characters ,necessity. The greatest possible being is God.

 God as a necessary being
Greatest possible being by definition should be a necessary being. By necessary being we mean a being who is not dependent . A being who is free and not contingent on others. So by definition , the greatest possible being must be a necessary being.
 If someone is not a necessary being ,then he cannot be the greatest possible being , because it would be dependent or contingent  on something/someone and that something/someone would be the greater than the being claiming to be the greatest possible being.
A necessary being/entity exists in all possible worlds.Examples include numbers , shape definitions and absolute truth.
A necessary entity cannot be false or fail to exist all possible worlds.

The concept of possible worlds in modal logic
Possible worlds :  Any hypothetical situation is called a possible world .

Construction of the ontological argument for God's existence
1. It is possible that a maximally great being/greatest possible being exists(God) .
2. If he exists , then it's true that he exists in some possible world.
3.As a necessary being , if God exists in some possible world , then he exists in all possible worlds,because by definition a necessary being exists in all possible world as he is omnipresent.
4.If he exists in all possible world , he exists in the actual world.
5.Therefore God exists.

In short ,
1.It is possible that a maximally perfect being(God) exists .
2.Therefore God exists.

This only applies to maximally possible being , and  it cannot be applied to anything lesser(unicorn).

This is Anselm's ontological argument.

For more
1.http://www.youtube.com/watch?v=RQPRqHZRP68&feature=endscreen
2.http://www.angelfire.com/mn2/tisthammerw/rlgnphil/ontological.html