Rust is the metal (Fe for iron) turning to the oxide (Fe2O3 normally for iron although there are other oxides).

FORBES: Will Handling Your iPhone 5 Damage It? Maybe...

Rust is visible on the metal parts and there are signs of wear and tear.

BBC: Worn and rusting rocket launcher still capable of killing

This is great news for website designers, Internet entrepreneurs, registered nurses, and masseuses in California, but grim tidings for traditional metal bashers in the rust belt manufacturing states like Michigan, Indiana, and Ohio.

FORBES: Welcome to the Michael Jackson Economy