Different computer mice have different levels of functionality. Some functions are built for specific games. With The R.A.T 7 mouse there is a sniper button which lowers the DPI in the case that you are playing a first person shooter and want to increase your accuracy. This function, and all other functions are usually controlled by the si gs65 bundled with the mouse.With most good computer gaming mice you will be able to have multiple profiles in which you can alter everything including macros, quick commands for faster game play, to button mapping. With the Razer Naga Epic Gaming mouse there are 12 buttons on the left of the mouse that can be specifically mapped to cast different spells when playing games like World of Warcraft or Everquest. So basically the functionality of a mouse is highly dependent on the firmware that comes with it and how well that firmware lets you change the use of the mouse buttons.
Mouse buttons and DPI are the two most important things to consider when choosing the ideal gaming mouse for you. I personally love to play World of Warcraft so I chose a mouse with upwards of 17 buttons that can be remapped the way I want. However if you are playing a first person shooter like Call of Duty or Counterstrike the number of buttons is not as important as the DPI of the mouse. DPI stands for dots per inch and in technical terms translates to the number of number of steps the mouse will report to the computer when it moves one inch. For quick speed and fast shooting the higher the DPI the better, with some mice these days reaching up to 8600 DPI. However as I wrote earlier, when sniping or doing something that required better accuracy than speed it is important to choose a mouse that can scale its DPI down to a lower number like 800-1000 DPI for slower, more accurate movement.