Listening to vs listening for

What's the difference between listening to and listening for?

A lot of ESL learners have this question.

The answer is simple:

Listen to = what I hear

Listen for = what I want to notice

Here are some examples:

You listen to a voicemail, but you listen for the person's phone number.

The mechanic listens to the engine. He is listening for that weird sound you said you heard, but he doesn't hear it.

The doctor listen's to your heart. She is listening for any problems.

We saw the clouds and listened to the rain. We sat on the porch, listening for thunder.