PFC is the abbreviation for
Private First Class, which is both a
US Army rank and a
US Marine rank.
Army
PFC is the third rank in the US Army enlisted rank system. The associated pay grade is E-3. The rank - worn in many places but most often the collar - is composed of one chevron and one pica, or rocker. It looks like this, pointed side up: <).
You can enter the Army as a PFC if you have some college or meet certain other requirements.
There are two eligibility requirements for promotion from Private, E-2 (PV2) to PFC, as there are with any promotion: time in service and time in grade. The time in service requirement is 12 months, waiverable to 8 months. The time in rank requirement is 6 months, waiverable to 4, (I'm actually not positive about those last numbers, I'll have to check)
Marines
PFC is the second rank in the Marine enlisted rank system. The associated pay grade is E-2. The rank is composed of just one chevron, similar to the Army's PV2 rank: <. Again, that's pointed side up, worn on the collar of most uniforms.
Someone with this rank in either the Marines or Army would normally be addressed as "PFC Smith"; it is more formal to say "Private First Class Smith", and perhaps a little frowned upon, though still correct, to say "Private Smith".