What's the Difference Between "Hear" and "Listen"?

What's the difference between "hear" and "listen"?

1. We "hear" a lot of things, whether we want to or not.
2. When we "listen," we do it on purpose.

Examples:
1a. I hear the dog barking next door.
1b. I hear the telephone when it rings.

2a. I listen carefully when my mother speaks.
2b. Many ESL students need to improve their "listening" skills, but they don't want to take a class in "listening"!