A magnet is a material in which most of the atoms have magnetic moments in the same direction. In each atom, each of the electrons has a property called quantum mechanical spin, which generates a tiny magnetic moments in one of two directions: "up" or "down". In most materials, the electrons are in pairs with opposite spins, so the magnetic moments cancel out each other. But in certain materials such as iron, there is an unpaired electron in each atom which leads to a net magnetic moment, and furthermore, the magnetic moments of each atom tend to align in the same direction. These materials are called ferromagnets. Normally in these materials, despite the tendency for the magnetic moments to align, there are regions with different magnetic moments. But if they are exposed to a magnet, the magnetic field from the existing magnet causes the magnetic moments to align everywhere, turning the material into a new magnet.

Usually either iron or nothing. Magnetic fields are caused by moving electric charges. One way to make a magnetic field is to coil up a wire and attach it to a battery. Bar magnets are special pieces of iron in which the electrons orbiting around the individual iron atoms are all pointed in one direction (which we usually call this direction 'North" because North is the direction of the earth's magnetic field).

Magnets contain crystal domains of magnetic solids, that produce virtual currents which creates the magnetic field. That's about all I know about them and if that isn't easy to understand, it's because I don't understand them that well either.