Things can be charged, ions can have a charge, etc.-which essentially mean an excess or reduction of electrons. Protons have a positive charge-but since we define an atom by it's number of protons, it is the negatively charged electrons responsible for an atom's charge. But what is charge really? Yes it's a property for describing things, but why are electrons negative and positrons positive? What is it at the fundamental level that differentiates these things, causing us to see them as "positively" or "negatively" "charged"? Does it have something to do with spin? If so, then what causes the non-spinning protons to have a positive charge and an anti-proton to be negative?