A switch is a physical break in the pathway taken by the current. An open switch is tantamount to an infinite resistance in the circuit. Think of a hose feeding water to a sprinkler. If you step on the hose the constriction in the pipe is now that infinite resistance and current flow stops. Remove the resistance and current flows again.
Your other questions about why do magnets attract are very deep and fundamental. There are four fundamental forces in nature. They are, in order of strength: gravitational force, weak force, electromagnetic force, and strong force. You can easily test the first. Drop an apple. The apple is attracted to the earth, and the earth to the apple. Why? How can there be this action at a distance? Even Newton himself was disturbed by this. The electromagnetic force is another 'action at a distance' force that governs the interaction forces between electric charges. It is one of the rules that make the universe work the way it does. By the way, magnetism is a manifestation of moving magnetic charges. If there was no electric charge there'd be no magnetism. Iron, cobalt etc. just happen to be materials that display their magnetic properties at common temperatures because of there microscopic crystalline structure. Anything with electric charge is in someway magnetic. Even the neutron which has no net electric charge has a so called magnetic moment.
