Responsive Design is when when your design responds to the device that it's on. This is something really important nowadays because of all the different types of ways that you can access the website. You want to make sure that your website actually... In this case this website that I'm looking at Foodsense that is on my MacBook Air. But, let's say I'm on this website, Zero. In which it's an accounting software site, when I minimize the window you can see how part of it is left out. You can't see most of the website and a lot of it is lost because of the fact that they're design isn't responsive, which isn't a bad thing .
It's good to have your design be responsive, but even if you don't because you know, that these people are only going to access it from the computer, then you might not choose to have a responsive design. But in the sense of Foodsense, they actually thought, "Well what if someone's accessing it from the iPad?" So as I start to minimize, you'll see that a lot of the grid of the website is actually changing. You'll see the navigation go from the left to the top. You'll start to see the navigation go away as I go closer and closer in width, so that is basically set up for the iPhone. So it's basically making a design that's able to be accessed and accessed well all the way from the smallest device, which could be an iPhone or smaller all the way up to a large computer like an iMac 27 inch or even a projector. It's making sure that whatever your looking at it on responds well. It is in the code, usually it's something that you have to not only design but a developer has to make sure that if X height or if X equals this or Y equals that then adjust accordingly. Or call on this style sheet. So as you know, websites have CSS, style sheet so they'll call on different ones to make sure that it's adjusting properly.