Narration is a term that is used specifically in the context of telling a story. Anybody that is telling a story is, by definition, considered to be "a narrator." A description simply tells you what something looks like, is like, or what function something performs. If you look on Google for language exchange websites and find the entry for italki then there will be a brief description of the italki website listed in the Google search results. But nobody is telling a story with that, so it is a description of what the italki website is for. But If I wrote a story about my experiences at the italki website, I would be narrating a story.