What is the difference between the words "pilalla" and "raunio"?
I think I see where the confusion comes from - when you look up the English word ruin or ruined, both of these come up as possible translations.
The ruin you were probably looking for is pilalla, which is the general word that means that something is no longer usable:
Laitoin keittoon aivan liikaa pippuria. Nyt se on pilalla.
I put way too much pepper in the soup. It's ruined now.
Raunio, on the other hand, means a ruin of a building. Here's a raunio:
Picture by Hasan Cilingir
The word raunio is often used in the essive case, which is the case we use for states of being:
Rakennus on rauniona.
Rakennus on raunio-na.
The building be-3rd person ruin-essive.
The building is in ruins.