Toptal

Nowadays, your website will be visited by a wide variety of devices: desktops with large monitors, mid-sized laptops, tablets, smartphones, and more.

To achieve an optimal user experience, your site should be adjusting its layout in response to these varied devices (i.e., to their varied screen resolutions and dimensions). The process of responding to the form of the user’s device is referred to as (you guessed it) responsive web design (RWD).

Source: Toptal