ARIA (Accessible Rich Internet Applications) is a W3C specification that makes your HTML more accessible by adding the missing semantics (usually attributes) needed by assistive technologies such as screen readers. They can be:

roles: used in sliders, menu bars, tabs, and dialogs properties: describe widget characteristics such as draggable, required, etc. state: describe the state of an element, e.g. busy, disabled, selected, or hidden

Bootstrap uses the role attribute in their nav widget:

 <ul class="nav nav-tabs">
  <li role="presentation" class="active"><a href="#">Home</a></li>
  <li role="presentation"><a href="#">Profile</a></li>
  <li role="presentation"><a href="#">Messages</a></li>