As a convention, buttons should only be used for actions while hyperlinks are to be used for objects. For instance, "Download" should be a button while "recent activity" should be a link.
Button styles can be applied to anything with the btn class applied. However, typically you'll want to apply these to only a and button elements.
Standard gray button with gradient
Focus state for keyboard and accessibility
Add the disabled class to links and the disabled attribute for button elements.