I'm not really a jungle so here's a basic rundown from an outsider's perspective. Anyone else feel free to correct me.
Trailblazer is the item for farming. It becomes AoE against monsters, stuns, and gives back health and mana. If you have a lot of trouble clearing, take it.
Saber is the dueling item. It reduces the enemy champs damage against you, causes your AAs to do a bonus true damage DoT, and provides vision of the target so they can't hide. Take this if you wanna hunt down the enemy jungler.
Knife is for counter jungling. If used on a big enemy jungle monster it gets reduced CD, gives extra gold, and extra MS to escape. Usually not that good even when used properly.
Blade is generally for ganking. It slows the target, gives you extra MS, and does a decent chunk of true damage. Some ranged CC and burst to initiate when you pop into a lane.
Now for the enchants. This is a bit simpler.
Devourer is for heavy AA champs. Big magic on-hit, AS, and the ability to proc multiple on-hits and activate certain-number-of-AAs abilities faster through the phantom. For example, Udyr will shoot flames every 2 AAs instead of 3. But this only happens after you stack it to 30 by killing big monsters or champs.
Runeglaive is for AP junglers. It gives a Spellblade proc that does AoE against monsters and restores mana. Otherwise it's a less powerful but much cheaper LB.
Cinderhulk is for tanks. It's like Sunfire but trades the armor for %bonus health and does more damage to monsters. Pretty straight forward.
Warrior is for AD junglers that don't rely on AAs. Some AD, CDR, and penetration. Also very straight forward.
Hope I didn't mess it up too badly.