The term Juggernaut most often refers to a massive, inevitable force that crushes everything in its path. In Star Wars: The Old Republic, it's pretty much the very same thing: a massive, inevitable Force-user that crushes everything in his path. The Sith Juggernaut stands firmly on the front lines of battle, hurling himself bodily into danger and withstanding brutal punishment that would destroy less-hardy combatants. His fighting style also follows the same basic definition - slow but inevitable, a single lightsaber swung with hard, powerful strokes and paired with an offhand shield generator. The Sith Juggernaut is not a DPS monster like the double-lightsaber-wielding Marauder, but he can hit some respectable numbers, and what he lacks in punch he makes up in momentum.
It takes more than just pluck and determination to hold the front lines. The Juggernaut is also highly dependent on first-rate gear - heavy armor and shields that augment his ability to absorb punishment from his enemies. Armor is especially important, and particularly for Juggernauts who use the Immortal skill tree for tanking, because of the relatively-low DPS output of the class. This is a class that is meant to get hit, hard and often, and still be the last man standing.
The heavy armor adds weight and momentum to Force Charges.
There are essentially two paths a Juggernaut can take - tanking or melee DPS. Melee tanks, using the Immortal skill tree, need to stack Endurance to build up a massive pool of hit points. DPSers will want to prioritize Strength to give their melee attacks extra punch. It's important to have lots of both stats, regardless of the Juggernaut's spec, but tanks will want more Endurance and DPSers will want more Strength.
When the character first starts getting orange Custom gear, tank-spec Juggernauts will want to use modifications (armoring, mods and hilts) of the Guardian type, which are high in Endurance, plus some Strength. DPS-spec Juggernauts will want to use Might modifications, which are higher in Strength, plus some Endurance. These items are relatively inexpensive, available from Modification Commendation Vendors on every planet (except Korriban). Armoring and hilts typically cost 7 planetary commendations each, and mods and enhancements cost 2. Constantly upgrading orange custom armor with the top modifications available will keep it viable as long as you like the look of it.
Juggernauts tend to swing their lightsasbers like bats. Here's one hitting an infield fly-ball.
Staying in top form, gear-wise, does not necessarily mean maxing out a crew skill (Synthweavers make heavy armor for Juggernauts) or lurking on the Galactic Trade Market for bargains. Good-quality gear can be purchased on every planet from two vendors: the Specialty Goods vendors, who sell their items for credits, or the Equipment Commendation vendors, who trade for planetary commendations earned by completing missions and as rare loot drops. Korriban, the Sith starting planet, has only a Specialty Goods vendor, who has only 1 item appropriate for low-level Juggernauts and charges 8 Korriban Commendations for it.
Level Range: 1 - 10
This is the starting planet for Sith Warriors and Inquisitors, a dry, hostile place pocked with ancient tombs. There is only the Specialty Goods vendor on this planet, standing by the elevators near the market area just outside the academy entrance, and he charges 8 Korriban Commendations for the one item that's useful for Warriors. It's only Medium armor, so it will likely be replaced fairly quickly once the Juggernaut advanced class is chosen.
Level Range: 10 - 16
The Sith Empire's capital world has a much better selection of specialized gear. The vendors here sell heavy armor accessories - boots, greaves and handgear - of the Avenger and Vindicator variety. They are cosmetically similar, but the Avenger items are geared more towards DPSers (more Strength) and the Vindicator items are more for tanks (more Endurance).
Level Range: 16 - 20
There is no specialization on this war-torn core colony world, but the Indignation pieces available from the vendors are good quality and will likely be a step up from anything you found on Dromund Kaas. The headgear comes from the Specialty Goods vendor and is fairly pricey. The other items come from the Equipment Commendation vendor.
Level Range: 20 - 24
Again, there is not much specialization on the Smuggler's Moon in Hutt-controlled space, but the Sovereign Mind items are suitable for either tank or DPS spec and are worth picking up. The Specialty Goods vendor sells shield and power generators - tanks will want the shield, DPSers will prefer the power generator.
Level Range: 24 - 28
The great, sweltering desert planet offers one of the better-looking chest pieces for Juggernauts, reminiscent of Darth Vader's armor in the movies but with more grey. Some might find it ironic that the hottest planet in the galaxy sells armor that's practically guaranteed to cause heat-stroke. Be warned that the cape on this and other cape-included pieces will behave strangely at times, but it looks badass.
Level Range: 28 - 32
While you're busy fomenting insurrection and promoting civil wa, be sure to stop by the markets near the spaceport to shop for accessories. The Specialty Goods vendor sells implants for tank and DPS spec, and the Equipment Commendation vendor sells boots, belts and trousers with a subtle purple hue.
Level Range: 32 - 36
The toxic wasteland of Taris brings back the specialization in terms of gear. There are 2 varieties of heavy armor here: Force Battler, which works best for tank spec, and Force Champion, which is better for DPS. Hit up the vendors for decent belts, pants and gloves.
Level Range: 36 - 37
Quesh is the planet where Darth Baras's plans are laid bare, but other than that there's not much reason to stay here for very long. It's another toxic swamp, but much smaller than Taris. The vendors sell ear pieces - the exact same ones available on Hoth - and armguards. Quesh is the start of the Paragon-Vindicator paradigm - Paragon stuff is generally suited for tanks, Vindicator stuff for damage-dealers.
Level Range: 37 - 41
The galaxy's icebox has some very decent gear, following the same Paragon-Vindicator paradigm as Quesh but with a better selection of items. The earpieces here are the exact same as the ones on Quesh, and they are all the Specialty Goods vendor sells.
Level Range: 41 - 44
Hoth may be the galaxy's icebox, but Belsavis is the galaxy's cooler for a couple of different reasons. It's slightly warmer than Hoth, with its isolated pockets of temperate regions surrounding volcanic vents, and it's also a prison planet. The Paragon-Vindicator specialization continues unabated here, and the Specialty Goods vendor offers an assortment of offhand devices (shields, generators, foci).
Level Range: 44 - 47
Back in Hutt space, Voss is the home of the bug-eyed, peaceful Voss species and their rivals, the lizard-like warrior Gormaks. As your hunt for Baras embroils you in the conflict between the two species, you earn commendations which can be spent on great late-game accessories and off-hand items.
Level Range: 47 - 50
The vendors at Coronet spaceport offer a number of upper-body items that will take you well into endgame. The Paragon-Vindicator theme holds true here, and there are some excellent armguards, gloves and chest armor here, as well as off-hand items.
Level Range: 50
Ilum is, for lack of a better term, the endgame planet. This is where you go after you complete your class story and become a Darth, and where you start doing daily missions to accrue a large amount of Daily Commendations (instead of planetary commendations, of which Ilum has none). Instead of an Equipment Commendation and Specialty Goods vendor, there are two Mission Support vendors here. The one guy sells artifact-quality implants and the other guy sells armguards.
Crushing one's enemies is a Sith's birthright, and no one does it quite as crushingly as the Juggernaut. And while some might say that "the best defense is a good offense," the Juggernaut knows to back that up with top-quality armor and sturdy shields. After all, if a person is going to go around being offensive - force-choking fools who give him sass, for example - he'd better be able to defend himself.