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In Benjamin Franklin’s experiments, he came up with the convention that we use today to define a “positive” charge. As it turns out, electrons, discovered much later, are negatively charged according to the convention. Lots of chemical and physical reactions involve electrons as charge carriers, so lots of physical phenomena have this weird opposite thing going on. E.g. electric current or “conventional current” flows in the opposite direction of electron current. Chemical reactions are also weird. Reduction reactions involve a reduction in electric charge, but gaining an electron. The model works just fine, but it can be tricky and/or annoying at times.

adding to abnorc's excellent answer - circuit diagrams are all drawn as if charge carriers are positive (this is called "conventional current"), but because electrons are negative, this can get very confusing when you're dealing with components where the flow of charge is one-way only (diodes, transistors, batteries, photometers...)