"Remember the Shabbat to keep it holy."
You can work during the six weekdays and do all your tasks. But Shabbat is the Shabbat to G-d your Lord. Do not do anything that constitutes work... It was during the six weekdays that G-d made the heaven, the earth, the sea, and all that is in them, but he rested on the Shabbat. G-d therefore blessed the Shabbat day and made it holy."
Shmot (Exodus) 20:8-11
"You must still keep my Shabbat. It is a sign between Me and you for all generations, to make you realize that I, G-d, am making you holy. [Therefore] keep the Shabbat as something sacred to you. Anyone doing work [on the Shabbat] shall be cut off spiritually from his people... Do your work during the six week days, but keep Shabbat holy to G-d... The Israelites shall thus keep the Shabbat, making it a day of rest for all generations, as an eternal covenant. It is a sign between Me and the Israelites that during the six weekdays G-d made heaven and earth, but on Shabbat, He ceased working and withdrew to the spiritual."
Shmot (Exodus) 31:12-17

You may do work during the six weekdays, but Shabbat is a sacred holyday to G-d, when you shall do no work. Wherever you may live, it is G-d's Shabbat."
Vayikra (Leviticus) 23:3
"Observe the Shabbat to keep it holy, as G-d your Lord commanded you. You can work during the six weekdays, and do all your tasks, but Shabbat is the Shabbat to G-d your Lord, so do not do anything that constitutes work. You must remember that you were slaves in Egypt, when G-d your Lord brought you out with a strong hand and an outstretched arm. It is for this reason that G-d your Lord has commanded you to keep the Shabbat."
Dvarim (Deuteronomy) 12-15

Shabbat Work
"The categories of work forbidden on Shabbat... sowing, plowing... baking... weaving... tanning hides... writing... building... kindling a fire... and transferring from domain to domain."
Talmud, Shabbat 73a
Why is carrying a needle out into the street considered "work," while dragging a heavy sofa across the room is not? What work is forbidden on Shabbat?

The Torah simply commands that "Six days work shall be done, but on the seventh day there shall be to you a holy day, a Shabbat of rest to G-d." A single example of forbidden "work" is cited:
"You shall not kindle a fire... on the Shabbat day."
Shemot (Exodus) 35:2-3

So how are we to define "work"? The Hebrew word employed by the Torah, "melacha", actually means "creative work." Thus, writing a single word is a melacha, while dragging a heavy sofa from one end of the room to the other is not.

This is in keeping with the reason that the Torah cites for observing the Shabbat:
"It is an eternal sign between Me and the children of Israel that in six days G-d made the heavens and the earth, and on the seventh day He rested and He was refreshed."
Shemot (Exodus) 31:17

Obviously, G-d did not sweat and toil to create the world; His "rest" and "refreshment" on Shabbat was not relief from exertion. Rather, for six days G-d created, and on the seventh day He ceased to create. So we attest to G-d's creation of the universe by ordering our lives in the same manner: six days of creative involvement with the world, followed by a seventh day of disengagement from the material and the cessation of all physically transformative activity.