... or what exactly is charge?
You will have noticed some effects of static electricity I'm sure in your life. The shock you get after sliding down a plastic slide, or the way your hair just won't lie flat after brushing it some mornings. Static electricity is all about the build up of charge on insulated objects.
So what is charge? It's a fundamental property of matter which causes a force between two particles. There are two types of charge, positive and negative, most things, like a normal atom, have a balance of positive particles and negative particles and so can be said to be neutral, or uncharged.
Unlike gravity, which is always attractive, electrostatic forces can be attractive and repulsive. We say that like charges repel and unlike charges attract, you've probably heard someone say that opposites attract in your life and this type of truism, usually stems from some universal law like this one. So rather like north and south poles of a magnet, positive and negative particles will feel an attractive force and positive and positive or negative and negative particles will feel a repulsive force.
These forces can explain the effects of statics, like a balloon sticking to a wall, or your hair standing on end when you are charged by a Van der Graff generator. I'm pretty sure that this accounted for medieval people believing in the power of wizards, that they were just good at these tricks, hence the big hats, clocks, staffs and lots of hair.
This is what power supplies do to the charge in wires, they exert forces on them. The free electrons in the wires feel the repulsion of the negative side of the cell and feel the attraction to the positive side, this causes them to move around the circuit. This is what we call electrical working, the forces causing charged particles to move through distances.
Once the charge is moving, we no longer call it static electricity, we call it current electricity. One of the most violent examples of the effects of static is lightning, we get charge building up in the clouds which is then suddenly conducted through the atmosphere as a lightning strike. In the clouds we have static, in the lightning we have current. Now you know what a charge is, it's time to begin to analyse that flow of charge that we call a current.