The number of pixels horizontally and vertically defines a monitors resolution in pixels or dots per inch (ppi or dpi). The greater the resolution, the more information or image you'll be able to view at once.
2560 x 1440
Response time is measured in milli-seconds (ms) and represents the time it takes for one pixel to go from active (black) to inactive (white) and back again. Needed to adapt to the constantly changing image on the screen. The lower, the better.