What makes a spoiler

I'm grateful that writers typically highlight "spoiler alerts", so that I can choose whether or not I want to keep reading before it's too late. But recently, I've been wondering what actually constitutes a spoiler. Most people would agree that divulging a key plot point that is only revealed after the climax is a spoiler. But, depending on how die-hard of a fan you are, smaller details may change the way you experience a story.

For example, book covers typically reveal how relationships between key characters will develop well into the story. Knowing these details before reading the story changes how I experience it. But, is that a spoiler? I don't know. Sometimes I feel they are, and other times I don't.

All in all, I find it hard to share what stories are about without accidentally sharing a spoiler.