Hi margaretHofer. Welcome to the forums.
Responsive design is a new trend, and is spurred on by the proliferation of mobile devices. Big, wide, cluttered layouts are not really good enough any more as more and more people are accessing content on small screens. Responsive design is one answer to this.
It was more or less introduced by this article a few years ago: http://alistapart.com/article/responsive-web-design/
Which spawned this book, which is worth reading: http://www.abookapart.com/products/responsive-web-design
There's a ton to read online, too, so time to get reading!