The concept of the Trinity comes from 1 John:
"For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one."
I know it sounds strange. Three can't equal one, right?
Many analogies have been posed to explain this. Some people say that God is like water; water can be liquid, solid, or gaseous, but it always remains water. But this is inadequate, for it would make God seem like one Being with three different forms over history. We know that the personalities of the Trinity are separate because Jesus often spoke about corresponding with His Father.
Another analogy that is often posed is that of an apple. Apples are composed of skin, meat, and seeds, but it is all part of the same apple.
This also fails. True, together these three parts make up an apple, but they are only part of an apple by themselves. Each person of the Godhead is God, not just "part of" God.
So what is God? There is a relatively simple explanation: God is one "what" and three "who-s". But this can be hard to grasp.
The best analogy that I have heard to describe God is as follows:
Space is composed of three dimensions: height, length and depth. Each of these are space; if there is height between two objects, there is 3-D space between them, if there is length between two objects, there is 3-D space between them, and if there is depth between two objects, there is still three-dimensional space between them.
But without any one of the three dimensions, the concept of space is meaningless. Space becomes either a vertical or a horizontal plane when deprived of one of its elements.
And still, each dimension is space in and of itself.
I'll close with a verse from Ephesians that has an interesting connotation when viewed in the light of this analogy:
". . . that you, being rooted and grounded in love, may be able to comprehend with all the saints what is the width and length and depth and height --- of the love of Christ.In Him, D3