Kosher wine is made just like other table wine, with an extra set of rules to make it consistent with Jewish dietary law. In order for a wine to be deemed kosher (Yiddish for "proper" or "fit"), it must…
Be made under the supervision of a rabbi
Contain only kosher ingredients such as the yeast and the fining agents
Made with equipment that has been certified by a rabbi to make kosher wines
Be free of preservatives or artificial colors
Be handled from start to finish by Sabbath-observant Jews
Producers of Concord-based wines (such as Manischewitz) that are sweetened with corn syrup must produce special "kosher for Passover" bottlings which are labeled as such
Some kosher wines are heated as part of the production process. These wines are known as ‘mevushal’ which literally means the wine was ‘cooked’
Kosher wines will have certification symbols. One of the most common looks like a ‘U’ in a circle Ⓤ, suggesting that it’s certified by Union of Orthodox Rabbis. A ‘K’ in a circle Ⓚ indicates it’s kosher, a ‘K’ in a circle with a ‘P’ Ⓚ-P is kosher for Passover, a ‘K’ in a star, cRc and Hebrew ‘רכש ’ are also indicators that the wine is kosher.
So, in summary, kosher wines don’t necessarily taste any different from non-kosher wines, or have higher or lower quality; they simply must be produced in accordance with a set of Jewish dietary laws.
L’Chaim! Cheers!