Unless stated otherwise, all information herein is taken from The World of Robert Jordan's "The Wheel of Time", Chapter 1.
The Great Pattern is the whole of existence and reality, past, present, and future, spun from the lives of all by the Wheel. There is a separate Pattern for each of the seven ages, the Pattern of the Age, which can only be partly changed by those whose lives are the threads. The only forces known to be outside the Wheel and the Pattern are the Creator, who created the Wheel, the One Power, which drives the Wheel, and the Dark One, who was imprisoned outside of the Pattern at the moment of creation. The Pattern weaves with no regard to the lives of those affected, but occasionally, it may need correction, in which case it will create one or more ta'veren, who influence those around them until the world is set on the correct path once more. Ta'veren cannot destroy the Wheel or completely change the destiny of the Pattern, they can only alter it. According to Moiraine, Min's ability to see auras is seeing parts of the Pattern (TEotW, Ch. 15).
"As the Wheel of Time turns, places wear many names. Men wear many names, many faces. Different faces, but always the same man. Yet no one knows the Great Pattern the Wheel weaves, or even the Pattern of an Age. We can only watch, and study, and hope." (Moiraine to Rand, The Eye of the World, Chapter 2).
"The Wheel of Time weaves the Pattern of the Ages, and lives are the threads it weaves. No one can tell how the thread of his own life will be woven into the Pattern, or how the thread of a people will be woven. It gave us the Breaking of the World, and the Exile, and Stone, and the Longing, and eventually it gave us back the stedding before we all died. Sometimes I think the reason you humans are the way you are is because your threads are so short. They must jump around in the weaving."
"The Wheel of Time weaves the Pattern of the Ages, and the threads it uses are lives. It is not fixed, the Pattern, not always. If a man tries to change the direction of his life and the Pattern has room for it, the Wheel just weaves on and takes it in. There is always room for small changes, but sometimes the Pattern simply won't accept a big change, no matter how hard you try. You understand?" (Loial to Rand, The Eye of the World, Chapter 36)
"The Pattern puts everything in its place precisely, and when we try to alter it, especially if ta'veren are involved, the weaving changes to put us back into the Pattern as we were meant to be" (Verin, The Great Hunt, Chapter 44).
"The Wheel weaves our lives to make the Pattern of an Age, but the Ages themselves are woven into the Age Lace, the Great Pattern. Who can know if this is even the tenth part of the weaving, though? Some in the Age of Legends apparently believe that there were still other worlds - even harder to reach than the worlds of the Portal Stones, if that can be believed - lying like this. The warp and the woof of the weave. Perhaps the Wheel of Time weaves a still greater Pattern from worlds. Well, that is neither here nor there. In all of these worlds, whatever their other variations, a few things are constant. One is that the Dark One is imprisoned in all of them.”" (Verin to Egwene, The Dragon Reborn, Chapter 21).