Summary
The Frame Size of the the image in a digital camera is directly connected to the pixel count both horizontally and vertically. Some frame sizes are referred to by their vertical pixel count such as 1080 HD. Other frame sizes are referred to by numbers close to their horizontal pixel count such as 4K Ultra HD (@4000 pixels across)
Frame Size
There are dozens of frame sizes that have come into existance through 100 years of film rolling through movie cameras.
Now in the digital age, there are no shortage of frame size options and very few standards for how those frame sizes are to be acquired.
There are some standards for how the media is to be shown because of monitor pixel counts, but even those are more like guidelines rather than standards.