Difference b/w trader and broker

Hi,

I have an interview with this commodity brokerage firm that focuses on OTC markets and especially crude oils. The job title is junior broker. I have a few questions.

1) What's the difference between a commodity broker at a brokerage firm and a commodity trader at, say, an investment bank commodity trading desk?

2) Is starting out career at a commodity brokerage firm good? Any possible exit opportunities?

3) What do you guys think about the learning curve?

4) How much would you expect to earn in the first year as a junior broker?

Thanks.