Not surprisingly, the juice made from squeezing grapes.
Comes in many varieties based on the type of grapes used. As is the case with wine, you can make pale grape juice from red grapes simply by extracting the juice and taking it away from the dark skins quickly. Therefore, you can get juice in colours ranging from pale yellow to dark red, nearly black. Grape juice is usually quite sweet, but this does vary depending on the grape variety. You can also get sparkling grape juice.
Many religions allow the use of grape juice as an alternative to wine. In the UK, a long standing law allowed grape juice manufactured for Churches to be VAT exempt. The law was then extended to any grape juice made for sacramental use - so for example Kosher grape juice often is marked this way on the bottle.
Orthodox Jews will only drink grape juice which is certified Kosher, in the same way that they will only drink Kosher wine. The most commonly given reason for this is precisely because so many religions use grape juice for their worship, it would be wrong for a Jew to drink grape juice which was intended for the worship of another religion.