There's some confusion about what schnapps is, what cordials are, and what liqueurs are. This situation is exascerbated by some benign misuse in the commercial alcohol market, as well as regional interpretations that can be quite different. So while I can offer some technical definitions, don't go complaining to your local liquor store that some bottle or other is mislabeled...
For years I labored under the mistaken impression that "schnapps" was just another synonym for "liqueur." Indeed, more than once I've been asked what the technical difference between a schnapps and a liqueur really was, and not only could I not say, I couldn't find any books that could say, either. Finally, someone came along and gave me the real story. Many thanks to Albert Grimm for setting me straight.
In his own words, Schnapps, a German word, "is the generic term for all white (clear) brandies distilled from fermented fruits. True Schnapps has no sugar added and is definitely an aquired taste, particularly for nationalities not used to raw distillates." So schnappses are different from liqueurs on two major fronts, they being both fermented and distilled, where liqueurs are simply fruits steeped in an alcohol which has already been fermented and distilled. You will also hear the words eau de vie in the context of liqueurs. I believe (though I'm always happy to be corrected) that this is a French expression for an unsweetened fruit brandy, very similar in nature to Schnapps. It has come to be used to mean an unsweetened liqueur as well, probably because of the similarity of taste and texture. But the original meaning was most likely as a brandy. Anyway, most significantly, this means that the run-of-the-mill home liqueur maker will never be able to really capture the whole essence of many commercial schnappses. The chemistry just isn't there.
Now, this doesn't mean that you should throw your hands up and walk away from ever trying to make a decent copy of a true schnapps. Liqueurs do go a long way towards tasting like real schnapps, and can make some compelling tastes on their own. One of my original intents in making liqueurs was to copy peach schnapps. I can't make it perfect, but a simple peach liqueur with some strategic addition of glycerine can fool all but the most discriminating palates.