A good Engineer is the person who makes sure every detail of your song is captured and blended together with all the other parts, so they make the most accurate depiction of your creation possible. Tracking Engineers, working with the producers, are responsible for picking the best microphones and putting instruments in the best spot in the room to record your sound. The Mixing Engineer's duty is to make sure the cacophony of sounds in the recording blend together so every voice is heard. Sometimes they are the same person, sometimes not. It's up to you and the producer to choose the best Engineer for the job.
To hear samples of my Engineering, scroll down.