Google Pixel uses the new 12MP 1/2.3-inch Sony IMX378 sensor fo its rear camera. This is the same sensor size that was found in a typical digital camera. It's a different sensor than the IMX377 that was used in the Nexus 5X and Nexus 6P. This sensor utilizes Sony's Stacked BSI CMOS sensor technology.
The sensor features large 1.55-micron pixels and a surface area bigger than the iPhone 6s Plus by 59% due to the Stacked-BSI technology that I'll talk about in a moment. The IMX378 sensor size is 6.20mm x 4.65mm. The iPhone 6s Plus was one of the best performers when it was tested by DxOMark and titled as "Best iPhone camera tested ever" when it was first reviewed. This was back then, and since then new sensor technologies have been developed, although the iPhone 6s also uses a Sony sensor for its back camera, but of course new technologies means better performance. The new IMX378 also incorporates a phase-detection AF system (PDAF) and utilizes Sony's SME-HDR technology.
Let's first talk about the Stacked-BSI technology. This technology was designed to allow smaller sensors to be produced for a given resolution because more of the circuitry is put behind the photodiodes layer. In the conventional BSI sensor, there is still a circuit section that surrounds each pixel. That circuitry is moved to a separate layer structure underneath the pixel section in a layer called 'Supporting substrate'. This allows Sony to produce even smaller sensors given the same resolution and pixel size, or alternatively, use larger pixel that occupies the same amount of space. If used with the same resolution, this means even larger pixels, which helps improve image quality and the low-light performance of the camera
The Sony IMX378 sensor also allows the Google Pixel camera to shoot 1080p120 and 720p240 high frame rate videos. The new Spatially Multiplexed Exposure (SME-HDR) technology is a new technology used by Sony which allows the sensor to produce HDR frames at full resolution and full frame rate. Google was able to provide both the video framerate and HDR recording features due to the faster image processing of their new Pixel devices.
The IMX378 is also used on the new Xiaomi Mi5S smartphone, so we can also expect this phone to resemble the performance of the Google Pixel camera. This sensor helps the camera to produce images with optimal exposure with less underexposed or overexposed areas, more natural skin tones, capture a wider range of colors and produce images that are visually more appealing to the viewer.
All in all, it's a very advanced sensor that uses Sony's latest imaging technologies to help photographers take better-looking images and videos even in complicated lighting conditions and support high-frame-rate captures as well.
For more in-depth technical explanation I recommend reading this excellent article on xda-developer, which explains about these new technologies in more details.