The Ten Commandments is written twice in the Torah. The second time is in this week's Parsha, when Moshe is reviewing the Jewish people's forty year sojourn in the desert. One of the differences between the two lists of the Ten Commandments is the commandment, "Remember - Zachor - the Shabbos Day to sanctify it" as it is written the first time and this week's Parsha where it is written, "Keep - Shamor - the Shabbos Day to sanctify it."
"Zachor" is a positive commandment to say kiddush and havdala - perform actions that show Shabbos is a special day. "Shamor" is a commandment not to violate the Shabbos through prohibited actions that are classified as "work".
The two mitzvos suppliment each other. One can keep the Shabbos and not violate any laws, yet, it will seem no different than any other day of the week. Therefore, we are obligated to make it different than the rest of the week through reciting kiddush, havdala, eating three special meals and special foods.