A "600" or "Spirit Bonder" is a form of tank that combines Spirit Bond, Protective Spirit, and low armor to stay alive. The build used to be popular for solo farming, until Holy Wrath was changed on February 25, 2010.
Most 600s use starter armor and/or are wearing no armor at all for some body parts, but a complete set of 15 AR armor is usually still sufficient. The low armor level will make most or all hits to the character deal enough damage to trigger Spirit Bond's healing, while Protective Spirit reduces the actual damage taken to a manageable amount. The "600" name comes from the fact that it initially was believed necessary to have at least 600 health to make hits deal at least 60 damage after Protective Spirit and thus still trigger Spirit Bond. This is not true, Spirit Bond checks the damage dealt before Protective Spirit, and the build works well with any amount of health, even if its strengths - resistance to degeneration and damage dealt via Retribution/Holy Wrath - shine with high character health.
There are a great number of other skills that can be used. The most common since the October 25, 2006, update is Shield of Absorption. Alternating Shield of Absorption and Protective Spirit allows you to fight huge groups of creatures at a time, while using Spirit Bond alone in its current form will only allow for fighting 5 creatures in hard mode and 8 creatures in normal mode.
To maintain energy, Balthazar's Spirit and Essence Bond are almost always used. They allow for gaining energy every time the 600 takes damage. Blessed Aura is also an extremely useful skill, particularly when using Shield of Absorption. Life Attunement can be used to increase the healing gain from spirit bond. Spell Breaker is a commonly used elite as it prevents enchantment removal and other harmful spells from being cast on the 600. Mantra of Resolve is useful in areas where interrupts are common, so that vital skills are not interrupted, causing the death of the 600. "I Am Unstoppable!" works the same against knock downs. Alternatively, Ancestor's Visage and/or Sympathetic Visage can drain melee foes of adrenaline and energy, preventing them from using any skills but signets (and possibly doing damage via Famine).
Damage and other players
A 600 can farm solo using Retribution, Shield of Judgment, etc. They also can tank for basically any ranged damage dealers, be it a single SS necromancer or a full group of nukers. However, a typical and unique way of dealing damage is via Retribution maintained on the 600 by another player ("the smiter"). They will typically maintain several other maintained enchantments on the 600 and not do much else, making them ideal for heroes or secondary accounts.
Although not often done, some other classes can 600, for example the 600 ritualist build used for MQSC which relies on Vengeful Was Khanhei. The main problem is that non-monk builds will have trouble maintaining spell prevention from skills such as Spell Breaker and will have a lower duration for monk enchantments such as Shield of Absorption. These builds will therefore prove themselves to be more efficient in areas where spell prevention is a minor problem compared to the damage done with the skills of the other profession.