Real Dictionary

listen
convention: You say listen up when you want someone to listen to what you are going to say.
convention: You say listen when you want someone to pay attention to you because you are going to say something important.
verb: If you listen for a sound, you keep alert and are ready to hear it if it occurs.
verb: If you listen to someone who is talking or to a sound, you give your attention to them or it.
verb: If you listen to someone, you do what they advise you to do, or you believe them.

listen in American English
the act of listening
to make a conscious effort to hear; attend closely, so as to hear
to pay attention to by listening; hear
intransitive verb: to give attention with the ear; attend closely for the purpose of hearing; give ear
transitive verb: to give ear to; hear

listen in British English
to concentrate on hearing something