Traditionally, a newly hired pilot is placed on probation. During this one-year period, the company is evaluating whether they made a good choice by hiring this individual. The probationary pay is low, on the order of $20-25,000 per year. I say "low" because the candidate is already well qualified at this point. The second year pay is much better, probably $40-50,000. As your career progresses, a pilot normally moves from airplane to airplane. This progression is normally to larger airplanes. The pay is figured using a formula that involves the weight and speed of the plane (i.e. the payload). So, as you move to a larger airplane, the pay goes up. A "senior" co-pilot flying a large airplane probably makes $100-125,000. The promotion to captain is generally back to a smaller airplane and the pay is slightly more than a co-pilot on a large plane. The captain's pay goes up as they work their way up in size again and tops out at approximately $200,000 for a captain flying a wide-body jet on international routes.
Of course, this whole discussion is complicated by the current state of the airline industry. Delta and Northwest just entered bankruptcy and United and US Air have been in bankruptcy for a while. During bankruptcy, retirement programs are being reduced to the minimum and companies are reducing pay to stay in business. Northwest pilots recently took a 15% pay cut and now 25% more is being proposed.
The irony of this situation is that traditionally the more desirable job was to fly passengers instead of freight. The freight carriers are doing great and those pilots have not had to take pay cuts and none are in bankruptcy.