Answer:
   
   The Sabbath prohibitions involve 39 categories of "work". Well,
   actually, what is prohibited is "melachah", as opposed to "avodah",
   which would imply labor. These categories relate to acts required to
   build the Tabernacle that was used in the desert and the early days of
   the First Commonwealth, before Solomon's Temple. The Talmud deduces
   this rule because the mention of Sabbath rest interrupts the telling
   of the building of the Tabernacle that takes up much of the last part
   of Exodus.
   
   Philosophically speaking, the Tabernacle and Temples were microcosms,
   the universe in miniature. Therefore, resting from the acts involved
   in building the Tabernacle is a way to commemorate G-d's "resting" (if
   one can truly speak of One Who is beyond Time resting) from creating
   the universe. Hence, melachah is defined more by an acts creative or
   world-changing content than by the effort involved.
   
   This prohibition on travel is found in Exodus 16:29: "A person shall
   not leave his place on the seventh day". "His place" is taken to be
   the town/city where he began the Sabbath. This shows that one form of
   change is not being in the town where you began the Sabbath. A town
   (actually, township) is defined as a group of homes that are within 70
   amos (roughly 105 feet) of each other. Add to that enough to produce a
   rectangle aligned with the compass points; in other words, fill in the
   corners to make a rectangle with sides on the N-S and E-W lines. Last,
   the town includes a 2,000 ammah (3,000 ft or so) area around the
   rectangle. If one wishes to travel from one town to another, and if
   the two towns are less than 4,000 amos apart, you can establish a
   formal central point for yourself between the two towns by putting
   some food down in a spot between them. This is called an "eiruv
   techumim" (a mixing of surrounding distances). In a sense, you
   established the significant part of the prohibited change on Friday,
   allowing you to be within any city that is within 2,000 amos of the
   food rather than within that distance from your starting point.