In the simplest way, it is a grape juice fermented by a specific method.
Wine is made solely from grapes. If you use any other fruits eg. apples or raspberries, you can not call such liquor a wine anymore.
In general, to produce a wine, you have to add some yeast to the grape juice. The yeast will “eat” the natural sugar contained in grapes and produce an alcohol. When the level of alcohol will reach a certain critical level, usually it is c.a. 12%, the fermentation process ends. Beyond this level, alcohol would kill the yeast themselves.
Sometimes a wine is not sweet enough as a winemaker wish, so when a fermentation is finished, some of them add some sugar to make the wine sweeter. But it is not a natural way of production of sweet wine, expected by most of the consumers.