Premium Processing promises faster service for certain employment based petitions. The USCIS promises to respond to a petition within 10 working days - with either an adjudication or request for additional evidence. A pending application can be "converted" to premium processing, or premium processing may be requested with the initial petition.
Requesting premium processing involves another USCIS form - I-907 - and payment of an additional filing fee. ($1250 as of February 2013).
The type of applications eligible for premium processing may change from time to time. They may include Preference Petitions (Form I-140) and the Petition for Nonimmigrant Worker (I-129). Not all petitions within those categories may be eligible for premium processing.
There are times when premium processing confers an important benefit. There are situations where "faster adjudication" does not confer any benefit at all, and can even be a detriment to an employer or beneficiary. We advise our clients where we are aware of a clear benefit for premium processing, and where we don't believe there is any advantage.