To make it very simple, the higher the generation the clearer the view and the farther you can see clearly. For example the 1st and 2nd Generations are limited on distance you can see and they are not real clear. In fact they are usually very grainy and not clear if that is a correct way of describing it. You might be able to make out a figure at 100 or 75 yards but no distinct details. Some of these also have a real Green tint to the view. Like the old Vietnam Scopes if you have ever seen a movie that they were allegedly looking through one. With the better units they also usually come with an IR Illuminator which is invisible to the human or varmints eyes. But unfortunately to get a good 3rd or 3+ Generation you are looking from $3500 -$6000 for a good NV weapon Scope. So I would ask what are your plans for it. There are other ways to go if you are for example night hunting for varmints.
The ATN Company also produces some decent NV. Of course there is Litton, NIVISYS and others. So certainly depends on your needs and for sure your budget! FOR SURE!
Bottom line! What ever you buy regarding NV be sure and get a written agreement if it is not what you expect from the product that you can say, return it within like 72 hours. But try it at night before you keep it on a very dark night. You may be very disappointed if you do not and have wasted your hard earned money on nothing that will do the task for you.