A website's supportive layout for different screen resolutions of devices like mobile, tablet & desktop in landscape/portrait is responsive website design. This give you a look at your website how it look in mobile/tablet device.
Responsive Web Design
From the starting of web till the date there are many revolution in web designing. Now-a-days designing is much better than previous designing. Everything change according to change in technology. Mobile & Tablet devices market share become more than PC business, also all mobile device is internet friendly. Thats why web designing changes to target all devices in market.
So for compatibility of website in mobile devices, new technology used to optimize website. To optimize website for mobile devices first mobile friendly site made beside of website in a subdomain. But recently this is much better which automatically give mobile website looks in mobile devices by using fluid/responsive web designing in CSS3.
What is Responsive Checker ?
This will show you how a webpage looks in mobile devices both in landscape & portrait mode. Fuild/Responsive web design automatically fit into width of device instead overflow over width. It shows webpage in demo devices like iPhone, Android, Nexus etc. Simply this program simulate a mobile device webpage look for checking responsive of website.