Soured milk stays good for a long time (similarly to cheeses) - pretty much until mold starts forming. It is a common drink in Eastern Europe, and production is very simple - essentially "happens by itself": if you leave fresh, non-boiled, non-pasteurized milk in room temperature for a couple days, it turns into soured milk. It's used as ingredient for a few pastries, but the primary use is to drink it straight (after mixing the two fractions), e.g. as a drink served along with fried potatoes. Despite what English Wiki says about culture-farming etc, soured milk as a traditional drink really doesn't take any effort to make.
Now, if the milk was boiled, or UHT, or something like that, what you get is curdled milk. This is spoiled, from moment one, and the only practical use is production of casein-based homemade plastic. No kitchen uses at all.
The way to tell one from another - soured milk forms a pretty much uniform mass similar to thick yoghurt, or large chunks, with a layer of transparent whey on the bottom. Curdled milk forms tiny grainy structures, sometimes slightly elongated or curly. They differ in smell significantly too - soured milk has a clearly sour, rather nice aroma similar to kefir, while curdled milk has a definitely unpleasant, dull smell.