List of HIPAA transactions:

270 Eligibility Request
271 Eligibility Response
275 Claims Attachment
276/277 Claims Status Request and Response
277 Claim Request for Additional Information
278 Referral/Authorization Request and Response
834 Enrollment
835 Claim Payment/Advice
837 Dental (837-D), Institutional (837-I), Professional (837-P) Claims
NCPDP 5.1 Pharmacy

NPI = National Provider Identifier – a new unique identifier assigned to all providers. Current requirement is that by May 2009 all claims must contain this number. http://www.cms.hhs.gov/NationalProvIdentstand/