There is actually a difference if we're talking about "adaptive layout" vs "liquid layout". In adaptive layout, the layout is adjusted in steps according to a limited number of sizes (various screen sizes, usually). While in responsive layout, the layout is adjusted continuously to every possible width.[1] When we're talking about responsive design, most example uses liquid layout.

Adaptive layout, though, is arguably better for readability. Optimal readability involves having control of the line-length of a body of text, which can be achieved with adaptive layout. In the case of liquid layout, line-length varies wildly over every width change, which is sub-optimal readability-wise.

[1] http://informationarchitects.net/blog/responsive-typography-...