Suits of living armor (also Living Mail, Japanese: 動く鎧 Romaji: ribingu meiru) are entities fundamentally of the same nature as golems, in that they are inanimate objects which have been animated and given a form of life through the efforts of sorcery.
Living armors appear to be a much more obscure branch of magical constructs then golems are; Lina Inverse was unfamiliar with the concept upon her initial encounter with Nama, and only Zelgadiss Graywords, descendant of and raised by Red Priest Rezo, and someone with a particular interest in magic related to creating life, was able to provide information on them. Of greater worth still may be the fact that, whereas golems are usually of minimal intelligence at best, living armors are fully intelligent, to the extent that they are readily describable as having "minds of their own" and are even capable of speech. This would make them much more versatile than golems, as they are capable of understanding much more complex orders, and they are even able to act and plan on their own initiative.
In the anime series, the only living armor to have appeared so far is Nama, who is heavily implied to be Naga the Serpent after an accidental encounter with a Hellmaster's Jar. However, from Zelgadiss' comments prior to Nama's revelation of being a human soul transplanted into a previously-inert suit of armor, it would seem that Nama's particular attributes as a living armor are not unique to her special method of creation. Other suits of living armor with a less exotic origin have appeared in the Slayers Special novel series.
Living armors, at first glance, appear to be more fragile than ordinary golems - a simple Bram Gash is enough to literally blow one to pieces. But, not only are they capable of reassembling themselves once broken apart, they display a strong resistance to magic: Nama not only shrugs off a Flare Arrow and a Dig Volt effortlessly, but is unscathed by a direct hit from a Dragon Slave - although it needs to be said that most of these scenes were only played for comedy. Nama also displays the ability to move to other locations quickly by breaking apart and then directing her swarm of levitating components to move to the desired position and reassemble.
Living armors and dullahans can be mistaken for each other with considerable ease. Both are insubstantial entities that use hollow suits of armor as physical shells. Living armors are magical constructs, though, while dullahans are undead wraiths who bind themselves to their armor by choice.