What's the difference between a defense attorney and a prosecutor?

Both represent their clients and pursue their client's interest. The prosecutor's client is the people of the state and their interests, as expressed through the legislature, is justice. Thus a prosecutor is duty-bound to seek "justice." A defense attorney represents the client's interests as expressed by the client. Thus if the client wants a plea bargain, the defense attorney seeks a plea bargain. If the client wants a trial and a chance at acquittal, the defense attorney must seek that chance.