When you earn wages you can have some put into a pension plan. Those contributions you make from your wages can reduce your federal taxable income in Box 1 of the W-2 however they cannot reduce your wages for FICA taxes in boxes 3 & 5. Some states allow the reduction and some don't so that is reflected on the W-2 boxes 15-20.
Now ... the "employers contributions" are what THEY put into your pension plan are considered "non taxable employee benefits". They are not subject to FICA or federal taxes by you or the employer since they are not considered earned income. State rules vary.
