Resolution is a main parameter of display and normally it is showed by the quantity of pixels. For example 320*240.

It means that there are 320 horizontal pixels and 240 vertical pixels. 

Above figure is usual resolution and their alias name.

Maybe you notice that sometimes the resolution is marked as 320(RGB)*240. Why?

As we know we use RGB color model and use one red, one green and one blue pixel to form a color pixel. At the same time we have different RGB matrix arrangement. So 320(RGB)*240 means that the RGB matrix is horizontal bar.