What is the difference between a REC/GO and a carbon offset credit?

A carbon offset credit is a transferrable verified and certified tradable instrument representing an emission reduction (or removal enhancement) equivalent to one metric tonne of CO2. In contrast, voluntary RECs and GOs are a tradable instrument recording the generation of one megawatt-hour of electricity (net) that has been delivered to the grid. RECs/GOs cannot validly be used as carbon offsets because they do not correspond to GHG reductions (see questions Is “additionality” relevant or necessary for RECs and GOs to be used in consequential GHG accounting? and What are the “environmental benefits” or “attributes” associated with RECs and GOs?). For a detailed discussion on instrument options, their environmental integrity, and how to properly claim emission reductions, see: Other Instruments for Claiming Emission Reductions.