What is the difference between commercial pilot and major national airlines pilot?

A private pilot is licensed to fly for pleasure or business. This is an entry level license, which requires less than 100 hours flight time and demonstration of basic knowledge and skills.

A commercial pilot can fly for hire. This would include instructors, crop dusters, glider tow pilots, and some charter pilots. It requires more than 200 hours flight time and more basic knowledge and skills.

An air transport pilot can fly airliners. This is a much more complicated license requiring a more stringent physical examination, and some very demanding flight examinations. Each airliner a pilot flies requires a type rating.

There are other ratings pilots can obtain, including an instrument rating and a multi-engine rating. Pilots may also obtain glider ratings if they are so disposed. A commercial glider pilot would likely be an instructor. There are also certified flight instructor ratings of various types.

That's more than you asked for. Sorry. It's been a while since I've had to deal with these things, so I am not up to date on the numbers of hours for each rating, but I think you could find those in wikipedia if you need them.

By the way, I hold a commercial pilot's license with an instrument rating, but am not currently flying. A license is not valid unless you also have a valid medical certificate. So my license and rating are not valid at this time.