Well, there is a dramatic difference in the number of hours required to obtain the commercial certificate as opposed to the ATP. Us lowly commercial pilots need only 250 hours (sometimes less if you train under 141) compared to 1,500 for the ATP. There are a few other miscellaneous hour requirements in there for the ATP too (X-C time, etc...) Also you need only be 18 for the commercial, 23 for an ATP. One interesting thing is that to get your ATP, the FAA requires you to be of "Good Moral Character"!
PIC's for 121 carriers are all ATP's, while the FAR
's require you to be a commercial pilot to act as SIC. Both certificates allow the holders to fly for compensation or hire, and actually training for F/O's is usually done to ATP standards just as it is for captains. Many airline F/O's, especially at major airlines, are already ATP's, having been captain on another aircraft type, or their employer requries them to have an ATP before being hired, or they get their ATP as part of training.