Why is the Sabbath considered a day of rest?
The requirement that we rest on Shabbat is explained by the Torah according to two broad themes. First, God "rested" from the work of Creation on the seventh day; therefore, we rest on that day to acknowledge God as the Creator of the universe. Second, we rest on Shabbat as a reminder of the Exodus from Egypt, our redemption from slavery. Our rest, a dignity not granted to slaves, reminds us that we are in bondage to no human master; we therefore acknowledge God's liberating power in our lives and in the history of our people. An intrinsic feature of rest (m'nuchah) is the prohibition against doing any manner of work (m' lachah), a prohibition that the Torah mentions no less than six times. The Torah never defines the concept of "work" in precise terms. That task is accomplished by the Rabbis of the Talmudic tradition.
The definition of "work," they claim, is derived from the Torah itself. One of the biblical verses that prohibits m'lachah on Shabbat occurs immediately prior to Moses's instructions to the people on the construction of the desert Tabernacle. From this fact, declares one midrash, we learn that the Torah defines "work" as any and all creative activities necessary for the building of that shrine. This teaches us that Shabbat takes precedence even over the construction of the holiest of structures. Over the centuries, the details pertaining to these restrictions have been expanded to include activities that were certainly never contemplated by the authors of the biblical and early Rabbinic sources.