In the past, the only Kosher wine available was for sacramental use. This was the heavy, sweet wine that many older Jews of today remember from their childhood. However, according to the strict laws of Kashrut, all alcohol made from grapes (ie Wine, Brandy, Sherry etc) must be made under the supervision of a Rabbi, and in conditions as JohnnyGoodyear mentions above.
Despite this fact, though, at least in the UK, a number of Orthodox Jewish Kashrut authorities would in the past permit "regular" wine at otherwise strictly Kosher Weddings, Barmitzvahs etc. (I use the term "regular" as most wines, even if not supervised Kosher, have no actually non-Kosher ingredients in them).
Nowdays, though, this isn't permitted, and indeed there are many excellent Kosher wines from around the world. This could be either cause or effect! The most common places to find Kosher wine from are:
- Israel - has an ever growing number of wineries. The Carmel vineyards have been producing wine since before the foundation of the State of Israel in 1948, under their own brand and under the "Palwin" brand (short for Palestine Wine). Most of these wines in the past were not so good. But more recently, they've had to catch up with other more modern vinyards and have produced some decent stuff as well. There are other wineries all over the country, such as the Golan Heights winery, the Barkan winery and many more. Due to the soil and climate, especially in the north, some of these wines are as good as any French wine.
- Europe - Most mainland Europe countries make wine. There are some pretty good Kosher wines from France, Spain and Italy, and no doubt other countries. There is also a lot of Kosher wine produced as "Vin de table" - ie not particularly good, but pleasant enough to drink on a daily basis, as the French in particular like doing.
- USA - There are Kosher wineries in California, producing wine under the Baron Herzog label.
- Australia - Australia has recently come into the Kosher wine market, under the Teal Lake label.
There is now no reason to shirk at the idea of Kosher wine.
Those from Israel and other countries have won awards in international wine competitions, and, while somewhat more expensive than "regular" wine, still provide a very pleasant drink.
Oolong mentions that isinglass is used as a fining (filtering) agent in some wines, and he is correct that this would make them non-kosher. Also, some very cheap red European wines in the past were coloured with bull's blood - definitely not kosher!
The issue is that even vegetarian wines can't be considered Kosher as it goes deeper than that. As wine is used for religious purposes in many reasons, there is a risk that someone making the wine offered a small amount to another G-D, and that would create large issues for religious Jews. Hence not only are the ingredients kosher for kosher wine, all the people making it should also be observant Jews, or at least under constant supervision of them.
There is also one opinion that says wine has to be kosher to prevent Jews indulging in alcohol with people other religions, as this would lead to inter-marriage etc. However, I don't hold much by this, as if it was the case, then all alcoholic drinks would have to be kosher (which they don't), and grape juice wouldn't have to be kosher (which it does).