A utility class does one job: p-4 sets padding, text-center centers text. Bootstrap 2,007 has utilities too (Positioning Utilities and Outlook Utilities), but treats them as helpers around its components. A utility-first framework makes them the main way to style: you build a card, a navbar or a whole page from utilities and write custom CSS only for what they cannot express. CSS Architecture compares utility-first with BEM and other CSS architectures.