In programming terms, a library is a grouping of code that provides common or additional functionality. Libraries consist of one or more files that expose objects and functions. Within a program, a developer includes or calls the library to use these additional objects and functions.
Modernizr enables developers to use some of the more advanced features of HTML5 and CSS3 in older browsers. It uses feature detection to determine whether a given browser supports a certain widget or effect and provides an alternative means of accomplishing the task at hand.