Hebrew term which means literally the Holy (Kodesh) Cupboard (Aron)! Often translated simply as "The Ark".
The Aron HaKodesh is essentially that - a cupboard in the Synagogue. What makes it special is its contents - the Sefer Torah, the handwritten scrolls of the Old Testament that are read during the services in Judaism. It can be of any size - some Synagogues may only have 2 or 3 Scrolls, some may have more than 20. It is usually lit inside.
In Orthodox Synagogues, it is usually in the wall that is towards Jerusalem, as Jews pray facing Jerusalem. (If you are in Jerusalem, it will point towards the Wailing Wall). Non-Orthodox Synagogues are often less strict on this.
The Aron HaKodesh is often very ornate, decorated perhaps with marble or carved wood. It usually has outer doors that can be properly locked (a Sefer Torah costs over $20,000 and some Synagogues have many of them). These doors will be usually be opened at the start of the service, and left open until the end of the service. There is then a curtain which is opened and closed as required. The curtain is often also very ornate, and may be changed depending on whether it's Shabbat or a Yom Tov.
Most people in the Synagogue will stand up while the Aron HaKodesh is open as a sign of respect for G-D's law as is written in the Sefer Torah. Usually it's opened to remove the scrolls and then closed until they are replaced after having been read from. There are also various places in the prayers, especially on Rosh Hashanah and Yom Kippur, when it's opened, and again one should stand. Various people are given the honour of opening and closing the Ark.
Often an appropriate quote is written or engraved above the Ark. Some common ones are...
- Ki Mitzion Tetzeh Torah O'D'Var Hashem M'Yerushalaym - For out of Zion will go forth the Law, and the Word of the Lord form Jerusalem
- Mah Tovu Ohalecha Ya'akov - How good are your tents, Jacob
- Dah Lifnei Mi Atah Ohmed - Know before who you stand
There is also often a representative engraving of The Ten Commandments above the ark, usually consisting of the first two words (in Hebrew) of each one.