When I played Heretic and Heretic2, I didn't think much of what is an elf since I took for granted that the main character is a human like us. One day I looked "elf" up in my English-Chinese dictionary curiously. It said an elf is a short small human-shaped creature. I began to be confused becos the Ht2 intro movie shows that the main character is a tall big guy. But Lord of the Rings also "proves" that an elf is a small humanoid.

Then I have some questions about an elf:

1, What is an elf or a sprite? Are they humans?

2, what's the use of their pointy ears? Do they have exceptional hearing?

3, I've found lots of games and books of European culture background have "elves" in them. Is there a literary tradition for such thing?

Well as far as I know there is no real ONE definition of an elf, the use of such a name probably only really came in use after the LOTR was published.

From what I cant tell I think they're mainly supposed to be immortal, though they can die from wounds or like in LOTR from a lack of a will to live(Grief from the loss of a loved one). They have great eyesight and are excellent archers, They probably do have pretty good hearing as well. I don't think they're really smaller than humans, in fact they're probably quite all tall and lean. They are supposed to be quite beautifull as well. They're definitively not Humans, if you take the LOTR's view of them.

Well, Elves have quite a history to them in Europe. Afaik, Elves originated as a public superstition.
It was said that Elven women were extremely beatiful and that they would lure sailors to their deaths with their beauty and charm - in other words, extremely dangerous, otherworldly beings.
Elves were often characterized as human in appearance but thin, almost fragile, and they were rather ghostly, magical beings.

In our modern day and age, all sorts of new, interesting traits have been applied to them - there are many different versions of Elves and people have their own favorite version. I've never been much of an Elf fan and I do understand your confusion, because the truth is that there are many interpretations of Elves. In some interpretations, Elves are even taller than humans, while in others they are definitely smaller.

My favorite version of an Elf is a thin, smaller than human, but very agile creature with human appearance and with very sharp senses and were skilled in the arts of magic. However, I like to see them as more fragile than humans, but then they also live much longer than humans.

As for Corvus... well, in the game, he is pretty thin, but muscular, and that seems to be a pretty standard interpretation of an Elf hero, although Elves are often depicted as blond and very beautiful, male as well as female. Also notice that the other Elves in the game are incredibly thin.

Oh and yes, most Elf interpretations make the Elves have good hearing, hence their characteristic ears.

I hope I answered you questions satisfactorily.

They're kind of like Vulcans on hallucinogens.

look I found one!!

Lol, that thing looks more like a Dwarf or a Gnome than an Elf.

Definitely a Gnome.  

Personally I prefer Wizards of the Coasts' vision of the mythical races to some of the other variations out there.
Judge for yourself:
Male:

Female:

OMG! That female in the middle is more muscular than any FPS male hero!

a bit hairier too  
needs a back-wax

Remember to read the tags at the bottom of each picture as it denotes the race, so that you aren't mislead into thinking that all those are Elves.

But yeah, the Elves depicted there definitely have the appearance that I prefer with Elves. Shorter and thinner than humans with long, pointy ears.