An Eternal Light (Ner Tamid) hangs above the ark in every synagogue. It is often associated with the menorah, the seven-branched lamp stand which stood in front of the Temple in Jerusalem. It is also associated with the continuously-burning incense altar which stood in front of the ark (see First Kings, chapter 6). Our sages interpreted the Ner Tamid as a symbol of God's eternal and imminent Presence in our communities and in our lives.
Where once the Ner Tamid was an oil lamp, as was the menorah which stood outside the Temple in Jerusalem, today most are fueled by either gas or electric lightbulbs. There is at least one Ner Tamid, located at the Adat Shalom Reconstructionist Congregation in Bethesda, Md., perpetually lit via a photovoltaic power source (solar). The Etermal Lights are never extinguished or turned off.
Sources: Rabbi Scheinerman's homepage