The term Elves is an umbrella term for various races that have descended from the Proto-Elves, and meet certain other criteria.
It has been a subject of debate and much contreversy what is the exact criteria - other than being descended from the Proto-Elves - to classify a race as "true Elves", and the ultimate conclusion is biological distance.
As Wood Elves and High Elves look similiar enough to each other that interbreeding between each other is not considered hybridization (and does not have the traits generally associated with hybridization, like difficulties with socializing in parent societies), Humans and Orcs have deviated from the common Elven ancestors enough that interbreeding with the earlier two races is considered hybridization, resulting in Half-Elves and Half-Orcs. In other words, Wood Elves and High Elves - despite the linguistic and religious differences - can adapt to each other, and socialize in each other's environments as children, while Humans, Half-Elves, Orcs and Half-Orcs can't.
Dark Elves are another special case, as their skin colour and several other biological traits differ markedly from that of Wood Elves and High Elves, but still have the same long lifespan as their overworld cousins, and give birth to children that would not have any biological handicaps at socializing if it wasn't for mutual racism towards each other, unlike Half-Elves and Half-Orcs who find it challenging to socialize in Human/Orcish and Elvish societies alike.
Another interesting case is that of the Ice Elves inhabiting the north-eastern slopes of the Grey Mountains, who - despite their physical resemblance to High Elves - have an entirely different culture, language and religion unrelated to that of any of the Elven races, and their other biological traits make them a truly unique race. It is believed that the Ice Elves are not an Elven race at all, and are called "Elves" only because of their physical resemblance to the Elven races (Human-like appearence, pointy ears, heavy growth of hair on the head but very limited growth of hair everywhere else).