HSA

I have been doing payroll for a number of years and it seem to be getting very complicated - especially with all the pre tax items. I have a client that has set up Health Savings Accounts for its employees. First question - how is this reported on w-2's. There is a designation W for box 12 - I'm not sure I understand the definitions as explained on the back of the W-2. Question 2 I live in Pa. I understand the amount deducted for the HSA is tax deferred for fed w/h, soc sec and medicare and in Pa for state income tax and also in Pa for the local taxes. Can anyone enlighten me or tell me where I might might find info. I have looked at the IRS pub for this subject and it is even more confusing talking about employer contributions and employee contributions. Any help would be appreciated.