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.
'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.
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)