The internet has changed significantly since the turn of the century. One of the changes that has taken place is that people now view the web on multiple devices, all of which have different screen sizes and resolutions. This has made designing and maintaining websites somewhat problematic. A common solution from 2005-2010 was to make multiple versions of the site–so there would be the traditional site, a mobile site, and in some rare instances a tablet site. Pictures would be sized differently on each site and tables and other design elements would be slightly different.
Rather than making multiple sites, websites today are often created with a “responsive” template that allows a single site to resize itself to adapt to multiple devices. Using HTML 5 and CSS3 sites are created to automatically swap out larger images for smaller ones as the screen size is reduced, to shift table cells from horizontal to vertical, etc. You can test out a website to see if it is responsive by grabbing the edge of the page and pulling it over. As the browser page size decreases the responsive site will make adjustments often changing images, reorganizing layout, and sometimes changing the navigation buttons.
The sites we are now creating are responsive, so the layout will adapt to the width of any device (desktop, tablet, mobile). This is a feature you should expect of any web designer moving forward as your site will most likely be viewed on multiple devices.