any lawyers here.. what is entrapment?

Just in case anybody thinks I'm a 伸手黨 and tells me to find a dictionary. I know what the word means. The basic idea is that police cannot trick someone into committing a crime and then persecute them for it. In real life it gets a little more tricky. For example if a female cop pose as hooker then maybe its a crime if you go up to her and ask her the price but count as entrapment if she comes up to you and offers? It is very tricky and so if someone who knows law can help shed some light that will be very helpful to all of us.