Sometimes regarded as a nativized form of Middle Chinese達 (MC dɑt̚), but this is unlikely, since Middle Korean has a final cluster which is difficult to explain phonetically as a result of borrowing. However, it may be cognate with Japanese達(たち, tachi), which is also a pluralizer.
Korean does not grammatically distinguish between singular and plural nouns. Thus, while 사람들 (saramdeul) means "people", 사람 (saram) can mean either "person" or "people", depending on context.
들 (deul) is rarely used with nouns denoting inanimate objects. It is more often used with nouns denoting animate objects (people and animals), but then only when it is semantically necessary to make a distinction between singular and plural, or to emphasize plurality.