New toys, new jargon.
For a long time the only way to check out a web site was on a computer, and usually a desktop computer. As monitors became bigger and cheaper and wider, we built big, wide sites that looked good on them. Then laptops became more affordable, so we had to tone it down a bit. Then something unprecedented happened. There is a literal explosion of people visiting sites on a multitude of screen sizes and types of devices. Smart phones, super phones, tablets, "phablets", and the list gets longer. Now the responsibility falls on us to deliver content that can be easily read on all these devices. For a little while, we began creating different websites for those users, like m.mywebsite.com, for example. This works well enough, but has some big problems. Some smart folks took advantage of new browser capabilities, and a new term took the web by storm : "Responsive Design".
One page, multiple sizes.
The grand idea behind responsive design is that, if planned for properly, content can adjust and size and morph itself to fit nicely on any size of screen. You design a site to be flexible enough, and then everyone gets a great experience. It is a lofty goal, and not every style of site is well suited to changing format dynamically. With any new site build, or re-build, we always think about the mobile experience first, and plan accordingly.
Mobile is more
Different screen sizes is just one aspect that should be considered when thinking about our phone wielding visitors. We can also tap into things like the GPS, for instance. All of a sudden, we know something more about our user. We know what their location is. We can choose to serve different information based on that. Why would we ask them for a postal code, when we could just give them a link with directions from their current location? We could also order lists according to distance away from them. We could display a banner of a retail store that is in their vicinity. Tapping into the accellerometer feature, we could show a help window if someone shakes their phone in anger. And this is just the tip of the iceberg.