ASPX, including all its "special" features like postbacks and validators, ultimately renders as standard Html. So it's good to understand how certain Html components, like anchors, work.
A simple anchor just has an target page in the Href, look like so:
Clicking this anchor will always redirect to PageB.aspx. The Href property can also contain a method to call. For example, ASP.Net renders LinkButtons as anchors whose Href calls the postback method:
Anchors can also have an onclick event. You could have this method call a message box, set hidden fields, or do whatever else you needed. However, the Href will not be called if the onclick returns false. For example, you could create a "Do you want to save?" yes-no MessageBox like so:
Knowing the relationship between the anchor onclick and href lets you develop a more interactive and rich UI.