Responsive Web Design (RWD) essentially indicates that a web site is crafted to use Cascading Style Sheets 3 media queries, an extension of the @media rule

"Mobile First" and "Progressive Enhancement/Unobtrusive JavaScript" (strategies for when a new site design is being considered) are related concepts that predated RWD: browsers of basic mobile phones do not understand media queries or Javascript, and it is wise to create a basic web site then enhance it for smart phones and PCs — rather than attempt "graceful degradation" to try to degrade a complex, image-heavy site to work on the most basic mobile phones.

Browser detection and mobile device detection are two ways of deducing if Javascript and certain HTML and CSS features are supported, however Javascript libraries like Modernizr, jQuery, and jQuery Mobile that directly test for features/user agents are also popular.

Example grabbed from http://goldilocksapproach.com/

Text form wikipedia article on RWD