Quantum properties are not as easy to grasp as "classical" properties (like mass velocity and force) because we can have no direct sensory perception of them. We can see an object moving, how much space it covers in a given amount of time is a property called speed. Spin is one quantum property, which is tricky to understand because it doesn’t correlate exactly with any physical process (like distance covered in time for velocity) in space as we perceive it. We only began to understand it in 1922 when the Stern-Gerlach experiment showed us that spin was quantized and could only have two possible values. For now we can say that electrons come in two flavors, spin up and spin down, which we can represent graphically by arrows pointing up or down. In matters other than spin, electrons that make up the electric current (free electrons) are identical in every way.