F1 power steering, mechanical vs electronic

[djb]

Just read a report of BAR testing a new mechanical power steering unit as next year "electronic"power steering will be banned.

Anyone out there have a more or less laymans term explaination of the two, and specifically, how the electronic one works and also why it is being banned. (I realize the last part is probably hoping for too much, who can explain sometimes FIA tech descisions.)

thanks,