A: Hybrid means some variation of a site is being decided on the server, before content is delivered.
With pure responsive design, the server sends the same markup to every type of device. Based on a web browser’s characteristics, the pure site’s layout and behavior is adjusted to provide an experience suited for that device. You can build fast responsive sites that request different assets (images, scripts, etc.) that suit web browser characteristics, but many pure responsive sites today do not. Pure responsive design has a reputation for lackluster performance, but it’s really a symptom of poor implementation. There is some redundancy with pure responsive design, though, because it’s inherently planning for one code base to span many device sizes and capabilities. When the impact of this is minor, the great advantage is a site that can scale up or down seamlessly whether you rotate a device or resize your browser.
Hybrid responsive design can mean several things. Sometimes it involves no pure responsive design at all, and simply means serving totally different versions of a site to different devices. Sometimes it means the server detects the type of device requesting a page, and tailoring parts of the response design to that device’s specific characteristics. Sometimes it’s called adaptive design or responsive delivery or responsive design with server-side components (RESS).
In any case, hybrid means some variation of a site is being decided on the server, before content is delivered, and different devices receive different content. In the simplest case, your desktop site remains untouched and phones are served a mobile site instead. The main benefits are this happens invisibly without redirecting to an m-dot URL, and you can send only what that device needs without redundancy. The obvious downside is you’re maintaining multiple code bases to provide the same content and functionality, and there’s a tempting trap to cut content or functionality in the name of performance and simplicity.
Users don’t want dumbed down experiences on small screens, so there must be a deliberate balancing act between compromising for performance and striving for continuity across devices. You can achieve that with pure or hybrid responsive techniques. Pure responsive design takes longer to implement well, but is easier to maintain and grow. Hybrid is faster to implement with tolerable performance for users, but it’s less flexible without accepting the maintenance burden of building for multiple overlapping device classes. There’s no silver bullet, but a lot of good options and opportunities to blend the two approaches.