What is mouse DPI?

DPI is dots per inch, it also relates directly to the mouse READING, along with the movement on screen. So that said, if you use a 1024x786 screen resolution, using a 1000DPI mouse, it would 'technically' take 1 inch to move from one side to the other. But it also relates to how many 'screen shots' the mouse takes per second. So higher DPI = more screen shots per 'movement cycle' which translates into a smoother movement of your mouse, and yields a higher speed. You can also reduce your movement speed (most commonly done in the game you are playing) to adjust the higher speed. So if you go from a 400 DPI mouse with 4 sensitivity, a 1600 DPI mouse and a 1 sensitivity would give you the same speed, but higher precision.