First created by the German Corporation UberChem as their contribution to the super-soldier problem, a Juicer stands at the peak of human physical abilities, and like most paragons of fitness the secret to success is performance-enhancing drugs.
A Juicer can lift and carry four times the weight an ordinary man can. He can engage in any strenuous activity like marching, running or swimming for ten times as long before getting tired. He needs only half as much sleep as other men, and can go five days without before he becomes impaired. His reflexes are legendary, there are virtually no circumstances, including surprise attack from behind, that he doesn't have a good chance of dodging or parrying the attack. You can pick hurl a Juicer to apparent doom and it approaches certainty that he will flip, roll, or otherwise land perfectly. A Juicer can recover from any given injury four times as fast as another man, is highly resistant to most drugs and poisons, and is practically immune to pain.
And all they have to do is die.
Perhaps I should back up. The idea behind the Juicer was to create a chemically-fueled superman that would reach the height of human potential and maintain it indefinitely. To this end, a mini-computer and IRMSS nanite-reservoir is installed in the subject's neck, this serves to regulate his natural hormones. Then the external Juicer Harness is surgically bonded to the subject, consisting of drug reservoirs, injectors spaced along the body, and a second computer-IRMSS combo in the chestplate. See here:

This helps to monitor and control the subject for maximum efficiency. When he gets tired, appropriate stimulants or endorphins are administered, when he gets bored (it happens a lot) it's a mild euphoric. Anything and everything that helps him maintain his edge constantly. He can also inject himself with emergency trauma nanites by pounding his chestplate.
The end result is the superman described above, but there's a steep price. The subject WILL die 5-8 years after conversion. There's no save, no cure, no resurrection magic, the body gives out under the strain and that's all she wrote.
There is a chance of detoxing a Juicer, especially if you get to him in the first 3 years. After that the odds are not so good, and even successful detox is highly traumatic, as the Juicer is literally a pale shadow of himself without the drugs.
Still, anyone willingly accepting Juicer conversion is embracing nearly-certain death. Which is really why Juicers are dangerous, they have nothing to lose. Juicers can be bitter about their fate, nihilistic, or even happy-go-lucky and determined to savor every minute of life, but they are all career soldiers, if not before conversion then after, and will think nothing of ending your life. Everybody's gotta go sometime, right?
Juicers have their own subculture distinct from "squishies" including wannabe hanger-ons. Juicer bars can be... interesting places.
A Juicer harness usually contains enough of various drugs to last ten months or so. Refills are expensive, which is another reason (besides recouping the cost of conversion) most Juicers are elite mercenaries.
In the last year, what they call the Last Call, the Juicer develops a variety of symptoms including blinding headaches, nausea, convulsions, and coughing blood. These are usually considered a sign that it's time to seek glorious death in battle.

Like the Glitter Boy, the Juicer has been a core class from the start of the game, and has many variants. Just so no one says I forgot all about them.
Hyperion Juicers are speedsters with even more insane reflexes, able to run faster than 50 mph. They have half the endurance of normal Juicers and need to eat twice as much as other people.
Titan Juicers are mountains of muscle on top of mostly-metal bone. They can lift two tons easy and smash people into a pulp by accident, but their reflexes aren't much better than average peoples.
Phaeon Juicers are meant to be perfect fighter pilots. They have reflexes almost as good as the Hyperion, great situational awareness and resistance to G-forces, and solid bonuses to piloting anything. Their out-of-cockpit abilities aren't as impressive as the standard Juicers.
Mega-Juicers are new, and they combine MOM technology from the crazy with the Juicer conversion. Basically, latent psychic abilities are stimulated and channeled purely into physical enhancement. The result is tougher and stronger than the Titan Juicer, faster than some Hyperion Juicers (and not much slower than most) and has more endurance than even the standard. The downside is that they are prone to spontaneous combustion, on top of normal burnout.
Delphi Juicers, like Mega-Juicers, must be latent psychics. They gain moderate psychic abilities in addition to their Juicer physical strengths. The downside is they have an Eldar helmet clamped around their head permanently, and during Last Call they lose all control of their psychic powers.
A couple more really crazy variants exist, but I think that's all the high notes.