ruin means the broken parts that are left from an old building or city like Pompeii wreck means the left parts of destroyed or badly damaged vehicles or ships, especially ships

The above answers address usage as a noun, both words can be used as a verb as well. As nouns they mean something that is severely damaged, as verbs they mean to damage, break, destroy, etc. Wreck and ruin are synonyms with very similar usage. Personally I'd say their essentially overlaps, especially as verbs. I can't think of a situation where using one is appropriate but not the other. As nouns they are more distinct. "Ruins" can specifically mean physical structures in disrepair, like Randy and ring have indicated, but it can be used to describe anything. "Wreck" can specifically mean a vessel or ship which is destroyed or damaged, but like ruin can be used to describe anything. A ruined ship can become a shipwreck. A wrecked building can become a ruin. In short, they are pretty much the same thing. Sorry for the overkill haha

Ruin has more to do with something weathered by time. A wreck is associated with a crash or sudden event, like a shipwreck.