"-tur", "-tür", "-tır", "-tir"; "-dur", "-dür", "-dır", "-dir" are set of suffixes that turns a nominal word (noun based) into a verb in aorist tense in the sentence. They simply mean is / are. Hence;
Erkekler çocuktur: Men / boys are kids.
Yet, the plural form is also true here:
Erkekler çocuklardır: Men / boys are kids. (Note that the suffix adapted itself according to the last syllable and the letter of the word)
What's more, you can even drop the suffix for the first one: "Erkekler çocuk". Nevertheless, you can not do it for the second one.
After several weeks of dealing with the "-tur" suffix, I get to think that it is used when expressing things that are facts, mostly a general description of something. For instance, "an owl is a bird" is a fact, general, with little or no exceptions. But "the owl is hungry" is not a general fact, it is more of a description of a particular situation. I have found this suffix in the 3rd person of sing. and plural. Not sure if use in other persons too.