You might have learned at some point that all nouns in Russian are either animate (одушевле́нные) or inanimate (неодушевле́нные). Roughly speaking, living things are animate and/or sentient: студе́нт, ко́шка, де́ти; and non-living things are inanimate: стена́, телефо́н, письмо́. This subject has been covered on this blog before.