In 6 days G-d created the world. On the 7th day He rested. But Gd doesn't need rest. He is not human. Therefore, we have to understand it as follows: Each day of creation Gd was creating another Mask "The Masks of Nature." On Shabbat He rested. He rested from creating Masks. Shabbat is a special day where Hashem is not creating any masks. If we observe Shabbat properly we can enjoy a special closeness with our Creator that can only be achieved on Shabbat.
(I heard this concept on http://Torahmedia.com )