I have a 2811 router and i wish to upgrade the IOS to the most recent. Reaching the download page, i saw several options:Can anyone please explain to me the difference?Thanks.
It's simply a version of the major releases ie. 12.4, 12.4T and 15 (3 versions). If you want to see what differences there are you should use software advisor -
this allows you to compare 2 versions side by side so you can decide which one is the one you need. It's mainly to do with features needed.