Our Lord gives us the paramout definition of Himself in the Old Testament when He reveals to Moses His Name:  

"I AM who AM." (Ex 3:14)
 

But after becoming incarnate, Our Lord gave us a closer look at what makes him tick.  To wit, the Apostle John recorded seven instances in which Jesus used the words "I AM" to describe exactly Who He Is: 

1.  "I am the Living Bread that came down from heaven."
(Jn 6:51)

2.  "I am the Light of the World." (Jn 8:12)

3.  "I am the Gate." (Jn 10:9)

4.  "I am the Good Shepherd."
(Jn 10:11)

5.  "I am the Resurrection and the Life." (Jn 11:25)

6.  "I am the Way and the Truth and the Life" (Jn 14:6)

7.  "I am the Vine, you are the branches." (Jn 15:5)
 

John recorded one more instance, this in the book of Revelation, and it makes the perfect bookend to the revelation to Moses (I AM who AM):

"I am the Alpha and Omega, the first and the last, the beginning and the end."  
(Rev 22:13)

1.  He is pure spirit. 
2.  He is pure act.
3.  He is existence, itself.
4.  He is omnipotent.
5.  He is omniscient.
6.  He is Love. 
7.  He is Mercy.
8.  He is Justice.
9.  He is unchangeable
10. He is unmovable.
11.  He contains all the universe.
12.  He cannot be contained.
13.  He has no beginning.
14.  He has no end.
15.  He is not created. 
16.  He is always creating. 
17.  He has no passions.
18.  He is perfectly content in His three Persons.  
19.  He isn't lonely and never was. 
20.  He holds everything in existence. 

St. Anselm - "a being than which no greater can be conceived."

St. Thomas Aquinas - the Unmoved Mover, the First Cause, the Necessary Being, the Absolute Being and the Grand Designer.

First Vatican Council - 

The Holy, Catholic, Apostolic, Roman Church believes and confesses that there is one, true, living God, Creator and Lord of heaven and earth, omnipotent, eternal, immense, incomprehensible, infinite in intellect and will, and in every perfection; who, although He is one, singular, altogether simple and unchangeable spiritual substance, must be proclaimed distinct in reality and essence from the world; most blessed in Himself and of Himself, and ineffably most high above all things which are or can be conceived outside Himself.
 

Of his own accord and by his own power he made all things and arranged and perfected them; and his will is the substance of all things. He alone, then, is found to be God; he alone is omnipotent, who made all things; he alone is Father, who founded and formed all things, visible and invisible, sensible and insensate, heavenly and earthly, by the Word of his power. And he has fitted and arranged all things by his wisdom; and while he comprehends all, he can be comprehended by none. He is himself the designer, himself the builder, himself the inventor, himself the maker, himself the Lord of all.

Tatian - 

Our God has no introduction in time. He alone is without beginning, and is himself the beginning of all things. God is a spirit, not attending upon matter, but the maker of material spirits and of the appearances which are in matter. He is invisible, being himself the Father of both sensible and invisible things.  God is Creator of all that exists.
 

Although most religions throughout the world are based on the idea of a Creator (or Creators), they differ infinitely from textbook Christianity in that they, at minimum, do not accept the following Divinely revealed Truths:

1. God is a Trinity of persons. 
2. Jesus Christ is the Second Person of the Trinity, the Son of God, and therefore, God Himself.

But even certain sects of Christianity differ on the Trinity and Christ's own nature.  To understand better what others believe outside of the Triune God, here's a short list of major religions and sects: 

1.  Judaism -- God is the absolute one, indivisible, and incomparable being who is the ultimate cause of all existence.  He is a spiritual, but not physical, being. 

2.  Islam -- Neither a material nor a spiritual being, God is the absolute one, the all-powerful and all-knowing ruler of the universe, and the creator of everything in existence.

3. Hinduism - There is one god, Brahman, the eternal origin who is the cause and foundation of all existence. The other various Hindu gods (Vishnu who is the Preserver, and Shiva who is the Destroyer) represent different forms of the universal god Brahman. 

4. Buddhism -There is no belief in a personal god. Buddhists believe that nothing is fixed or permanent and that change is always possible.  "...all phenomena arise in dependence on other phenomena, hence no primal unmoved mover could be acknowledged or discerned."

5.  Shintoism -  A polytheistic religion that revolves around the kami, which are supernatural entities believed to inhabit all things.

6.  Zoarastrianism - Exalts an uncreated and benevolent deity of wisdom, Ahura Mazda (Wise Lord), as its supreme being.

7.  Christian Scientism - Founded in philosophical idealism, in which there is no material world, only the spiritual; and all disease is due to mental error.  God is Creator and Christ His Son, but Jesus is not Christ, nor divine, nor risen, and man's soul, per se, does not exist.  God is not a Trinity, nor is He a person.  

8.  Unitarianism - A Christian sect that believes God is not Trinitarian, but a single entity. Jesus was a wise teacher, but not God, nor God's Son.  Reject infallibility of Scripture and the doctrine of Original Sin.  

9   Mormonism - The Trinity is made up of separate and distinct beings, not three persons in One God.  Everyone existed in spirit before they were conceived.  Christ died for man's sins and man is obligated to follow his example in choosing good over evil.  Book of Mormon is proof of continuing divine revelation.