Screen resolution depends on two things – the number of pixels in the display, and the size of the display.
The categories HD, Full HD, 2K and QUAD HD tell you the number of pixels, as follows.
If you see Full HD+ this simply means larger than Full HD.
But this only gives you half the picture. The larger the screen, the more the pixels are spread out. If you know both the number of pixels and the size of the screen, you can work out the resolution in PPI (pixels per inch).
For example, Samsung Galaxy S9 Plus has a 6.2-inch Quad HD display - which works out at 529 PPI. By contrast, iPhone X has a 5.8-inch display with 458 PPI. But since the human eye is pushed to tell the difference in resolution beyond around 300 PPI, other factors such as the ability to display a wide range of colours are at least as important – if not more.
For more on touchscreens, see our Best for touchscreens page