Bootstrap tutorial

Buttons

The .btn classes are designed to be used with the <button> element. You can also use these classes on <a> or <input> elements.

css class description
btn Specifies that element is used as button.
btn-primary Specifies a button filled with a primary color. There are variations:
btn-outline-primary Specifies an outline button with a primary color. There are variations:
btn-lg Specifies a large button.
btn-sm Specifies a small button.
btn-block Specifies a block level button.

close button

btn-close and btn-close-white provide the default style for the close button.

states

You can set active state for non-button element by adding class active and the aria-pressed="true" attribute. 

<a href="#" 
    class="btn btn-primary btn-lg active" 
    role="button" aria-pressed="true">Primary link</a>

<a href="#" 
    class="btn btn-secondary btn-lg active" 
    role="button" 
    aria-pressed="true">Link</a>

For specify disabled state you can use the disabled attribute of button. For <a> you can use the disabled css class.

Primary link 
<button type="button" 
  class="btn btn-secondary btn-lg" disabled>Button</button>
<a href="#" 
  class="btn btn-primary btn-lg disabled" 
  tabindex="-1" role="button" 
  aria-disabled="true">Primary link</a>