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The moon pulls the ocean on the near side of the earth more than it pulls on the center of the earth. The pull on the ocean at the far side of the earth is smaller still. This causes the near ocean to accelerate toward the moon most, the center of the earth less, and the far ocean still less.
They occur because of the difference in gravitational effect on one side of an object (like Earth) compared to the other. Here's how they work (full explanation of tides): The ocean on the side of Earth facing the moon gets pulled toward the moon more than does the center of the planet. This creates a high tide.