What the crap is LU, NU, OU?
Basically, it's nerd-speak.
Derived from Smogon, I believe, which is a website dedicated to
competitive battling. Like, as anorakky as you can get. Full of weird strategies and calculations and optimisation. And they tier the pokemon depending on what's most useful.
Overused, OU, are the ones with the most applications. UU tend to be less strong, but still good at what they do. Underused. NU are considered useless competitively, which is why they're referred to as "Never used."
Thing is, the pokemon they consider useful are vastly different to the pokemon an average battler would consider useful. They're playing in a world where fire/flying pokemon are untenable because everyone uses Stealth Rock. Where nobody would use a Hypno when they could be using an Uxie. Where a point of speed makes all the difference.
It's very clever, and very effective, and they're far better at pokemon than I'll ever be, but it's incredibly insular. I'll refer to them for intelligent strategies and to give me a couple of ideas, but I'm never going to use Stealth Rock because not only do I not need to, but nor do most people who aren't playing against
other Stealth Rock users.
Smogon's good as a guideline, but a little mad. Persian was considered OU in the first generation, and ranked above Dragonite. Look at the reasoning and it makes some sense, but it's a very strange shifting scale.
Not to mention, I find it a little too clinical. Rather, a lot too clinical. I'd rather pants it.