Ruins are what remains of a house or some other building when it has not been looked after for a long time and starts to fall apart.
Ruins of buildings that are very, very old are often considered to be very interesting, and people will come from a long way to look at them.
Rust is what happens to iron when there is moisture around. Things will rust faster if there is any salt in the water, which happens often near the sea or if salt has been used to de-iceroads or sidewalk.
Rust is a distinctive reddish-brown. It is common to see old cars falling apart from rust.