(This is in response to a question from an ESL learner. To see that question go to //http://www.usingenglish.com/forum/ask-teacher/56415-4-more-questions.html)
We hear something by perceiving it with our ears. The act of hearing something requires no conscious effort on our part. However, to listen to somebody or listen for something does require a conscious effort on our part.
There are some occasions where we can break the rule. Non-native speakers shouldn't try to do it themselves -- it doesn't always work, and when it does it usually sounds old-fashioned. But you might need to recognise it.
In earlier times, "hear" could sometimes mean the same as "listen". There are some idioms that show this:
In the House of Commons, part of the British government, if members agree with a speaker, they may shout "Hear, hear!" -- this is an instruction to listen to the speaker. (A more modern equivalent might be: "What he/she said.")
The idiomatic phrase "I hear you" means you not only hear the other person, but you take their complaints or advice seriously.
In many translations of the Bible, even modern ones, "hear" is often used as a synonym of "listen". Jesus is quoted as saying, "He who has ears to hear, let him hear", and many Old Testament proclamations begin with a phrase like, "Hear, O Israel!"
Sometimes, especially in the military, important announcements may begin with the phrase, "Now hear this."
Even in modern everyday English, when giving somebody strict instructions, especially when we are angry, we might finish by demanding, "Do you hear?" This means something like, "Did you listen?" or "Do you understand what I said?" This isn't old-fashioned, but it's not very polite.