When your little brother breaks your carefully constructed Lego village into its tiny parts, he destroys it. To destroy is to completely demolish.
To destroy is the opposite of to construct or to build. When you destroy something, you cause destruction, when you build something, you are involved in its construction. In war, soldiers are sometimes sent on missions to search and destroy. This means their job is to find a target and completely take it out. Destroy doesn't have to be physical though. If you get an F in your math class, that could destroy your dream of going to an Ivy League university and too much criticism can destroy your self-esteem.