Shabbat is the marker around which we structure time in Judaism  -- from sunset on Friday to nightfall on Saturday, we rest refresh and prepare to relaunch.  

On Friday evening, Shabbat candles, placed in candlesticks, are lit to mark the beginning of each Shabbat and represent the two commandments Zachor (to remember the Sabbath) and Shamor (to guard the Sabbath and keep it Holy, as the 10 commandments instruct us in Deuteronomy).

After the candles are lit, Sabbath wine is blessed and shared with the family from a special goblet known as the Kiddush Cup -  a symbol of joy, fruitfulness, and s celebration.

It is also traditional to eat challah, each Shabbat. 

Havdalah marks the end of Shabbat and the beginning of the week.  It marks the separation between sacred time and secular time.