'Magnetic' Discovery May Reveal Why Earth Supports Life and Mars Doesn't- This invisible, protective shield likely existed shortly after the planet formed; a finding that could shed light on why Earth is habitable and Mars is not.
An artist's impression of the magnetic fields of Venus, Earth and Mars interacting with charged particles streaming from the sun. Earth's magnetic field protects it from these atmosphere-stripping particles.
Learn about magnets --for kids (This has a nice picture of the magnetic field) but it's description of how magnets make a magnetic field isn't correct. Better find another site to describe a magnetic field)