UML Sequence Diagram General Question

I was wondering what a Sequence Diagram has to display? Is it just relationships between classes or it shows the sequence of each class or both?

sequence diagrams show the messages being passed between classes in a timeline reading from top to bottom.

The diagram shows instances of classes as objects or actors at the top of the diagram, each object has a timeline running below it. Messages between timelines relate to operations or methods of the class.