(Actually, there is a 4th generation Eneloop battery that is easy to identify because it says "Panasonic" on the side, but these are not available in the US yet.)
If you've bought Eneloops over the years or you buy used electronics that have Eneloops already in them, you may want to tell the different generations apart. The main reason for this is to avoid mixing and matching different batteries. And that's important --not usually. You shouldn't mix different brands or models of batteries, but would you even notice if you put one version 1 Eneloop in with 3 version 3s?
Probably not, but when a device takes several batteries together, run-time will only be as good as the weakest battery. So those fancy new V3 batteries are now limited to the performance of a V1 battery. That's irrelevant in a TV remote, and and probably a small difference in an RC toy. But in a flashlight or a bike light, it could make a big difference in run time.
The different Eneloop AA batteries all have a code on them to identify the generation. The battery above has a code next to the crown. First generation are HR-3UTG, second are HR-3UTGA, and third generation are HR-3UTGB. The battery above is therefore a third generation battery. AAA batteries have a 4 in place of the 3, so a 2nd generation AAA has a code of HR-4UTGA.
In store packaging, 3rd generation Eneloops have "1800" in orange prominently printed on the blister pack.
Third generation batteries have significantly lower self-discharge, so they can last for years in your emergency flashlight and still have most of a charge when the power finally does go out. They also operate at very low temperatures --important for bike lights and other outdoor uses.
And, of course, the 3rd generation batteries provide more charge cycles. You may not think you need 1800 charge cycle, but a high charge cycle rating means the battery will maintain its typical capacity over a higher fraction of those charge cycles, even if you subject it to high current, deep discharge, low temperature, vibration --in other words, a typical day in the life of a battery that works for me.
Rechargeable batteries are now so good that you can just use them for years and replace them when they seem tired. But if you want, you can get more performance out of your rechargeable batteries by periodically reconditioning them. If that sounds like becoming a full-on battery otaku, it' actually not a big deal, but I'll save that topic for another day.