Pros:
Great Jungle
Great base damage
very tanky late game
Great ganks pre-6
Possibly the best ganks post 6
highly versatile with the ability to go master-assassin Nocturne and dominate, or go tanky and still destroy on damage.
can dive turrets at a low level with his spell-shield, and his ultimate.
fear
level 2 ganks

cons: early blue dependence
no slow without red
your ultimate makes everyone hug turret even if you're at another lane (can also be considered a pro if your teammate is in a duel)
early game is a little slow
delayed-fear

Nocturne plays the role of a high-damage off tank, or a assassin depending on what you need.

In most scenarios, Nocturne is a GREAT pick for 5v5 team ranked, but lacking a little in solo ques. for solo ques, I would recommend Mundo, Lee Sin, or Shyvana.

Start to finish of a route: Wolves > Blue Golem > Wraiths > Red Lizard > Plain Golem > Wraiths.

Do not forget you have GREAT ganks, even at level 2 if you get your fear (if you're level 2 and can't gank, don't get your fear and level up your E.)

With nocturne you can go many ways, Vamp Scepter, Doran's Blade, Cloth Armor, Boots (although your Q increases movement in ganks and can be used to get from point A to point B), although Regrowth Pendant isn't too good of a item.

It all depends, if you want to be aggressive early on I would go vamp scepter, if you want to clear your jungle very quickly with some sustain and get fed late game (I prefer option A) then get Doran's Blade: It gives you AD, and life-steal so you don't need to recall.

Boots, the rule of thumb: If you're going to be ganking early on (level 2-3 ganks) then you may choose these, although your Q gives you movement speed it may not be enough and this can be the difference between a kill and just making them recall, so if your teammates aren't CC heavy and can't stun them before your fear then get these.

Cloth Armor: more of a older style choice, not something I would see myself using just because nocturnes heavy base-damage can compliment a vamp scepter quite well.

Okay, so lets just say you are confused on which game to pick.
If you need someone who is going to need to be tanky, go bruiser.
Although you may not be able to assassinate someone as easily as my AD build, you will be able to still deal great damage, CC their carries to keep them in the fight and allow them to die easily, and will not have a problem with taking heavy damage, as well as a deflect CC / ultimates well.

Assassin Nocturne: You really are lacking on your ranged ADCs ability to take them out, and really need someone just to come in and turn the fight into complete chaos for the enemy team with 3 shotting their carry, then assassin Nocturne is the way to go with this.

As a bruiser, I would pick between 4 items: Infinity Edge, Bloodthirster, Last Whisper, and Guardian's Angel.

If you're needing that extra damage in team fights on the squishy ADC, go Infinity Edge.

If you need damage and sustain go with Bloodthirster, and if their team is quite tanky than I would go with Last Whisper.

If they are a heavy damage team and don't really have a tank, I recommend having your entire team get a GA.

In the jungle your spell priority will look like: Q > E > W

While gankin you NEED to fear them while not under the tower so it make look like: Q > E > W > R (R being generally used for a instance, heavy hitting dive while the turret will not hit you unless minions aren't there.)

In late game team fights it may look like: E > Q > W > R (W of course being used on the opposing teams carries.)

Feel free to PM me any time if you feel as though I missed out on something, I am extremely tired at the moment.

If you felt as though this guide helped you, please make a positive remark or vote.
If you don't feel as though my play style fits you don't down rate, only down-rate if you feel this overall is a horrible guide.