One of the most interesting things about великий могучий русский язык (the great and mighty Russian language) is its словообразование (word-building, morphology). If you have a chance, do pick up a copy of Roots of the Russian Language: Elementary Guide to Wordbuildling by George Z. Patrick.
Too bad there is no book like that that’d talk about приставки (prefixes) and суффиксы (suffixes) of the Russian language. Yet it is these two that make Russian language so выразительный (expressive), fluid and lively.
Consider the prefix со- or с- in its meaning of быть вместе (to be together) or делать совместно (do something together) – this is similar to the English “co-“:
Соавтор is a co-author as in писатель ищет со-автора для работы над биографией знаменитого политика (a writer is seeking a co-author to work on a biography of a famous politician)
Сотрудник is a co-worker or someone you трудиться (work) with. However, in Russian it also means “worker” in general as in сотрудник полиции (policeman), сотрудник учреждения (office worker) and научный сотрудник (research associate). The noun сотрудничество (cooperation) and the verb сотрудничать (to cooperate, to work with) are однокоренные слова (cognates) that also share the prefix.
Сослуживец is literally someone you служить (serve) with so that can be either a co-worker or a comrade-in-arms. Interestingly, the word соратник, that literally means a comrade-in-arms, is not usually used to describe someone you served in the military with. Instead, it’s used to describe a close associate. For example, Надежда Крупская была не только женой, но и верным соратником Ленина (Nadezhda Krupskaya was not only Lenin’s wife, but also his loyal associate).
Сограждане – used in plural only, this word means “fellow citizens” as in наши сограждане продолжают эмигрировать (our fellow citizens continue to emigrate).
Собутыльник - бутылка means “bottle” and собутыльник is someone you drink this bottle with. However, this word has a strong negative connotation unlike its mostly neutral translation, “drinking buddy”.
Собеседник is someone you have a беседа (conversation) with; it’s your conversation partner. Владимир оказался очень интересным собеседником (Vladimir turned out to be a very interesting conversation partner). By the way, a job interview is собеседование.
What other nouns with the prefix со can you think of?