The wires of this "circuit" are made up of copper. Copper is chosen because its atoms will "share" some of their electrons with neighboring copper atoms.
These electrons can then wander easily through the wire. Further, if they run into a magnetic field, that field will shift them all in one direction creating an electrical current. This process is known as "magnetic induction."
Here, so that we can see them, we represent electrons in the wire as big spheres with a minus sign.
