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.
Base styles
Standard gray button with gradient
States
Hover state
Focus state for keyboard and accessibility
Active state
Add the disabled class to links and the disabled attribute for button elements.