The resolution is the size of an image. Usually, resolution is denoted by width x height (in pixels). For example, a Mega Drive screenshot has a resolution of 320x224 pixels in NTSC regions, however, the resolution in PAL regions is 320×240. It is noted that not many games take advantage of the extra resolution, leading to borders around the top and bottom of the screen.
Display resolution refers to the pixels displayed on screen.
Overscan is the situation in which not all of a televised image is present on a viewing screen. This was typical of CRT televisions. In order to compensate for this, older game consoles and arcade systems usually rendered games at an overscan resolution that was higher than the display resolution.