Aatrox is one of thosee junglers who are fun to play, and can have alot of diffrent item builds.
Before season 4 started, riot nerfed his early game. they nerf his attack speed slightly when his blood well his high.
this might be a good time to explain his passive: When aatrox uses his abilitys he gets blood well, wich gives him attack speed. the second part of his passive is a built in guarding angle, when it is popped, you get more hp back depending how high your blood well is.
when you jungle aatrox, you wanna get attack speed and lifesteal.

AAtrox loves lifesteal, thats why lifesteal qunis are so good on him, with ls qunis you can start dorans blade and sustein in your jungle as long as you want. i run 2 lifesteal and 1 ad quin on him. if u want you can run 3 lifesteal.
Aatrox loves attackspeed even more, 9 attackspeed marks is a must, your lvl 3 Aatrox cant be stoped unless you get cc`ed or gang'banged.
As for your seals and glyphs, its pretty standard, armor and mr all the way.

The masteries are simple.
4 points in fury for the attack speed (witch makes you a god alone).
2 points in sorcery so you can spam them Q's and E's.
1 point in butcher, standard jungle mastery.
next you max your dmg with your last 14 remaining points.

now you should have 9 points left( if you dont have 9 points, either I suck at math or you ****ed up)
2 points in block
2 points in tough skin, again standard jungle mastery.
1 pint inunyielding
3 points in vetran's scars for that extra health witch Aatroxs loves.
1 point in juggernaut to make you even more unstoppable

you can go 9/21/0. it all depends on your team, if u have a squishy top and support, then yeah, i would go 9/21/0. but for the most time you will have a tanky top cause thats just how the meta is right now, and ppl loves to play taric, tresh and leons atm so for the most part you will have a tank in your team other than you.

When you use lifesteal qunis you can start dorans blade and 1 potion. this makes you scary as hell in the first 10 mins of the game. you can sustain in the jungle for ever.
on your first back, you should be able to afford a vamp sceptre and boots( remember to buy at least 1 ward every time you b!). the vamp sceptre will make you sustain even better and "tank" any dmg you take from your ganks.
you wanna get moby boots as fast as you can to run after ppl and waste less time in the jungle.
after moby boots you need to take a second and see what you are up against.
Who are fed?
are they ad or ap heavy?
are you more fed than they are?
after you have asked your self these questions,your next item will be either: giants belt, negatron cloak or a stinger.
most of the time i rush zephyr, it gives you tenacity and a lot of attack speed. this is probably your most important item.
depending on how your match is going, your 3 main items will be omen, spir visage and zephyr.
your finaly item will most the time be a GA. (basicly you can die twice and still be alive).

Alternative items for situations are firecape, thornmail, nijatabis for ad heavy teams.
banshees, hexdrinker for ap heavy.
BT, phantom dancer, ghost blade for when you are that far ahead and need tank at all.

start w and activate it to get your blood well going and dmg increase.(if you are invading or getting invaded, you need to start Q)
at lvl 2, get e.
lvl 3, get q.
max e first to get maximum slow and clear time.
max q second to max your dmg.

just a little about your W.
when active, you get increased dmg every third hit at the expense of losing some of your own hp.you will also get blood every third hit.
when not ativated, you get will heal yourself every third hit. this is increased dramatically when your hp is under half.
only have it active when your hp is above half.
if your hp is under half you shouldn't have it active.

Start what ever buff where botlane is.
go to the other one, watch out for invade.
kill either wolf or wraith.
Now here you have to watch your map for the rest of the match closely.
if all the lanes are pushed up, you have two choices.
Option one, continue farming jungle.
Option two, find the enemy jungler and stalk him. counter gank his ganks and get a double or triple kill.
Time all the buffs and ward enemy blue/red buff.( blue/red buff spawn time is 5 min)
When to dragon?
when enemy jungler is top or dead.
when enemy botlane is pushed up to their tower and cant get to dragon.
Remember to time the dragon and stand redy for the respawn( dragon respawn time is 6 min)
when you gank a lane, lets say mid, try and get behind the enemy and hit your e first if possible, then a few auto attacks before you Q em. if you land your full combo its a 100% a free kill. even if you only hit your Q they should be dead, unless they have armor or you are far behind.

poke with e as much as you can, land your q on as many ppl as you can is it is an AOE spell.
have w ative when your hp is full, remember to unactivate it when you are getting focused.