http://www.ef.com/english-resources/english-grammar/definite-article/ The word "the" is one of the most common words in English. It is our only definite article. Nouns in English are preceded by the definite article when the speaker believes that the listener already knows what he is referring to. The speaker may believe this for many different reasons...
Предлагаю почитать раздел: WHEN NOT TO USE "THE"
Правильно кто-то подметил, что зависит от контекста.