Premium processing can be thought of as an upgrade to certain non-immigrant and immigrant employment based visa categories.  The way that it works is that the sponsoring employer pays $1,000 to the US immigration service in exchange for a commitment to process the petition within 15 calendar days.  What does "process" mean you may ask?  It means that they will either approve, deny or request additional evidence within that time frame.  If they request additional evidence, then the 15 day clock restarts anew when they receive your reply for additional evidence.  The process is available for nearly all of the first three employment based immigrant categories (with the exception, currently, of multinational executives and managers).  It also does not apply to immigrant investors and immigrant religious workers.  Nearly all of the applicable non-immigrant employment based categories are covered.  Additionally, you can either file the premium processing request initially with your petition or you can upgrade it at any point.  It can be a real time saver.  The question to ask yourself is whether the time savings is worth $1,000.  Many times it can be.  For example, if an alien is in H-1B status that will expire in the next month or two and has an employment based immigrant petition pending, upgrading that petition through premium processing may mean that the alien can apply for adjustment of status and a work permit sooner and avoid having to renew his H-1B status and the costs associated with it.