The inventor and discover of magnetism was William Gilbert. He was the first to investigate this and found it out using scientific methods. He also found out that the earth is a giant weak magnet.
Who does it affect...
Basically, magnetism affects everyone because electricity without magnetism, nothing electric wouldn't work. So if you didn't get it at first electricity is nonexistent without magnetism and vice versa. It also affects all of us because our panets magnetic field.