Why is Golurk NU?

I agree Golurk is underrated but it's honestly fine where it is.

Firstly, speed. It's too slow to compete in the higher metagames where unless you have monstrous defense stats (which Golurk does not), all sorts of STAB super effective will easily put Golurk down.

It's typing is cool but pretty iffy defensively as well.

It's great in NU, possibly good in RU, I know it's iffy in UU and it'd be garbage in OU.

I think that some Pokemon are wrongly tiered, but Golurk isn't one of them. If you were just doing a random battle and you and your opponent had mixed tier Pokemon, you'd probably do fine with it. It probably would do fine with an RU team as well.
But against a planned out OU team with a decent trainer? Golurk would be rubbish. I agree with what Pokedra says.

If you support Golurk well, you can definitely use it in Doubles where Trick Room is more popular, easier to set up properly and actually works pretty well. It has great movepool (and abilities) to abuse TR: Iron Fist boosts the Power of Elemental Punches and Hammer Arm (the latter is even cooler as it makes you slower increasing your speed under TR) and No Guard that allows to get 100% accurate Stone Edge, Rock Slide and DynamicPunch (oh God, I love this move so much!). If you're worried about lack of pure power, pair it up with Cresselia or Musharna as both get Helping Hand and Trick Room (and are immune to EQ). the only thing that Golurk lacks is a reliable, hard hitting Ghost-type STAB.

Golurk is one of the few physical based Ghost Type we have so far. And what's the best physical Ghost move (bar Shadow Force)? Yeah, the fantastic Shadow Punch...Golurk doesn't even have access to Shadow Sneak, a priority move almost ALL Ghost Types have (Troll Freak again) .
That being said, Golurk is not by any means a bad Pokemon. It has a great ofensive typing, good abilities and stats (shame it has such poor speed...) and also a interesting movepool with support moves like Stealth Rock which gives it some versitality. In NU it sucks as a Spin Blocker (Wartortle demolishes it), ca't touch the most common physical walls (tangela, alomomola) but is also one of the few ghosts able to defeat the most common Trappers like Skuntank or Drapion. I personally think Golurk is one of those cases which turns to be a "better Pokémon" in higher tiers.

It would be useful to know it's moves and abilitiy

Honestly, Golurk lacks the speed to compete in the higher tiers.
If it had high enough speed, I am pretty sure it would move up to RU.

I agree with Gonz0, Arceus_17 and Pokedra. While Golurk may have a good type combination and a good movepool for abusing its Iron Fist ability, the thing that holds it back is the fact it has a lot of common weaknesses and it doesn't have the Speed of the top threats. Also, No Guard makes it get 100% accurate DynamicPunch which may seem tempting on paper, but the only way to get Golurk to shine easily is under Trick Room, and that can take time to set up. Also, Shadow Punch (its main Ghost STAB) doesn't even do all that much damage unless if it gets an Iron Fist boost. B2/W2 did expand its movepool a bit by giving it the elemental punches, so it's still a pretty hard hitter under the right conditions.

I've used Golurk in RU quite a bit recently and i can say for a fact that its pretty underrated in the tier at the moment, and it's definitely worth considering for a teamslot in RU. This is because of a combination of things. Firstly, Golurk's typing provides it with a good amount of very important resistances and immunities in RU. For starters, its completely immune to Normal-, Fighting-, and Electric-type moves, all of which are seen pretty often in RU, especially the latter. Outside of these immunities, Golurk also resists Poison-, Rock, and Bug-type moves, while the former isn't very important offensively, Rock and Bug sure are! Resistances to Stealth Rock, Stone Edge, Head Smash, Megahorn and U-turn are never a bad thing, especially in a tier with Escavalier and Aggron running around. Speaking of Stealth Rock, Golurk's aforementioned typing, decent bulk, raw power, and coverage make it a reliable setter of the move. Golurk can even spinblock its own Stealth Rock to an extent to boot! (although admittedly it doesn't have a good match up against Kabutops and Cryogonal :/, so blocking Rapid Spin is a little more prediction and risk based). Lastly, Golurk shares a lot of its resistances with former RU Pokemon Nidoqueen, making it one of the best options to replace her on teams where her Extra Speed wasn't particularly needed. I've also heard Golurk is quite decent in UU too, serving as a decent check to various common UU Pokemon such as Heracross, Mienshao, Raikou, and Snorlax (although it doesn't 100% counter any of them them ;-;), while still performing the SR+spinblock role, although i haven't tried it out there to see for myself.

On a final note, i'd like to bring up that Golurk is actually quite close to moving up to RU from NU at the moment, being the closest NU Pokemon to the 3.41% cutoff by far, so it might just end up out of NU by the end of the month.

Golurk is NU because the only thing it can do reliably is check low-tiered common leads in said tiers- meanwhile setting up its own stealth rocks.

Key-word: reliably. Sure, it's an amazing Pokemon with the right support, but you can't expect it to work everytime.

I've used Golurk in OU and the only build that was somewhat reliable was No Guard/Scarf and it still sucked compared to a useful Pokemon.

Also, tiers are based on usage, and not on opinion of strength or capabilities with hard support.

On the other hand, its a lot better on wi-fi doubles for the reasons Gonzo listed- but also because its immune to Fake Out, which is a huge blessing for an offensive Pokemon.