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.
- HD: 1280 x 720
- Full HD: 1920 x 1080
- 2K: 1920 x 1080
- 4K (also known as Quad HD): 4096 x 2160
If you see Full HD+ this means the panel displays the same 1080 pixels horizontally, but has more pixels vertically than Full HD because of the wider aspect ratio.
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 pixel density in PPI (pixels per inch).
For example, Samsung Galaxy S9+ 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 struggles 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 so.