Answer:
   
   Jews that closely follow traditional practice don't do a whole
   category of activities, known in Hebrew as Melacha, on Shabbat.
   Melacha is usually badly translated as "work"; but it is better to use
   the Hebrew word, because the English word carries connotations of hard
   labor and other concepts that are inappropriate. Technically, there
   are 39 supercategories of melacha. They are derived from the classes
   of activities performed during the construction of the Mishkan
   (Tabernacle) in the desert, following the exodus from Egypt.
   
   One of the prohibited forms of melacha is lighting a fire. So Jews
   that closely follow traditional practice do not light fires on the
   Sabbath. (In fact it is traditional to light candles just before the
   Sabbath comes in as the last act of melacha done before the Sabbath
   descends). Now, when you drive a car, and you put your foot on the
   ignition, you produce a spark of fire. Thus, such Jews also do not use
   motor vehicles on the Sabbath, for it violates the prohibition against
   lighting fire on Shabbat. Electricity is more complicated (there being
   long arguments about whether it is fire, or whether it is only banned
   for falling under one of the other heads), but without getting into
   the detail, suffice to say that it is not used because it too falls
   within the various prohibitions. However, there are some exceptions,
   such as lights or VCRs preprogrammed on timers (note that the VCRs,
   the screen should not go on, as this would entice one to watch the
   program).
   
   However, different movements have different positions on the issue.
   Reform Jews, and other Progressive ("progressive" is the term used for
   Reform Judaism outside the United States) movements tend not be
   concerned with Melacha at all.
   
   The position of Conservative Judaism is more complicated, as it
   attempts to reconcile modernity with traditional practice, working
   within what it views as halachic process. Some in Conservative Judaism
   hold that electricity is not a form of fire, nor does the use of
   electricity inherently violate any other Shabbat prohibitions. Thus,
   it is an acceptable opinion within the Conservative movement for
   electric lights, telephones and other electrical appliances to be used
   on Shabbat. Note that other prohibitions, such as the prohibition of
   cooking, remain. Others in the movement follow the traditional
   practices seen in Orthodoxy, and restrict the use of electricity in
   addition to other prohibitions.
   
   In the area of driving on Shabbat, the actual stance of the
   Conservative movement is stated in "Travel on the Sabbath: A statement
   unanimously adopted by the Committee on Jewish Law and Standards"
   which was affirmed unanimously on 2/17/60.
   
   This statement indicates that "the Sabbath cannot function as the
   great day of the Lord unless we consciously "make a fence around it".
   The most important of the fences we must make to safeguard the Sabbath
   as an oasis of peace and of holiness is the avoidance of travel."
   However, Conservative Judaism exempts a specific type of travel from
   the above limitation: travel to a synagogue for attendance at worship.
   This exemption was granted based both on the needs of modern
   conditions, were people live in widely scattered areas, as well as a
   view that it was an emergency measure which the individual might make
   when in his conscience he or she knows that no alternative exists,
   stressing the values that would be lost by travel even in such
   instance. Note that both opinions limit this exemption to the need of
   reaching the synagogue for attendance at worship. Still prohibited is
   travel for other ends, such as travel for social purposes, or travel
   to the synagogue for purposes other than to worship (for example, in
   order to attend a Bar Mitzvah ceremony or reception, for the
   motivation here is not the service of G-d but the honor of man).