First, let me tell you what I'm talking about when I say Elves, because the term has been applied liberally in reference to other similar fantastical races, but the better we constrain it here, the more accurate we can pinpoint at the characteristics of this group.
I'm talking of human-like supernatural beings of slender build, light colored-skin, graceful and good-looking, often conceived as being taller than the average human and capable of living much, much longer (if not forever) than humans. Those elves, well, they are the proud product of two main mythologies: Nordic and Celtic.
Within those mythologies, the so called Fair Folk, are artistically and magically gifted, invariably mysterious in their ways, evasive, remote and fans of nature. They're associated with natural landscapes, from millennial forest full of secrets, to mystical boglands covered in mist and myth. In most fantastic literature, writer's imagined elves (singular, "elf," from the Norse "Alfar," an ancient tribe of gods, people, or spirits that represents the origins of Nordic mythology, similar to the Tuatha Dé Danann of Celtic folklore) are regarded as the opposite of dwarfs. They are ethereal, of the air rather than the earth. Slender and graceful, rather than corpulent and forceful. Or, for those who like animals, elves are more like cats, while dwarfs are more like dogs.
Elves allegiance is with nature rather than any other race. They may work with humans, or against them, all depends on the cause. Only one thing is certain, elves rarely make loyal companions.
Interestingly, the arrival of Christianity turned elves into darker creatures. After the middle ages, stories of Elf Women temping men and even boys to follow them into certain dead—at times from sinking into the bog, a times from exhaustion as they made the men dance, and dance—became common; a way to distant people from their traditions and folklore.
Elves also started to mutate into smaller and more domestic beings. Like so, Brownies and Tomtes, Pixies and Nisses, and even bedtime-tales fairies are all distant relatives of the primordial elves, variant kindred, with the stress on the mischievous and domestic rather than the dignified and majestic attributes of the Elven Folk.
In literature, modern and ancient, elves have taken many forms, but as every time Norse and Celtic folklore is involved, it is J.R.R. Tolkien who has created the image most modern readers and movie-goers recall at the mention of this race.
Tolkien's elves are tall, aristocratic and wise, custodian of old knowledge, who lived in beautiful hidden realms untouched by time and decay.
All members of the elfdom had been portrayed in fiction. Elves as little creatures with craftsmanship inclinations are one of the most common, creatures like the elves working for Santa Claus at the North Pole or like those who night after night appear to finish the work the poor shoemaker could not. Beyond Tolkien's seminal work, portrays of the Elven Folk in its most traditional form are also abundant in fiction, from Shakespeare's play A Midsummer Night's Dream to Paolini's fantasy novel Eragon, elves have fought among and against humans, but always on the side of what it's decent and fair. However, much as dwarfs, they often fins themselves cast on the role of side-kick, which has helped to preserved their reputation as not always being loyal to one person or group, with Legolas from The Lord of the Rings as perhaps the most notorious exception to this rule.
-Pringle, David, ed. The ultimate encyclopedia of fantasy. Carlton, 2006.