The Term Race (Q."Lie" S."Nos"), was used in Middle-Earth with quite different meanings, according to J.R.R. Tolkien Elves and Men were technically one race as well as Halflings were technically a subrace of Man.Also Orcs were, for the most part, descendants of Elves or Hildor and as such part of the same race, while Dwarves and Ents, as they were not Eruhini but had different origins, were seen as something separate.Also Men, Elves, Orcs, even Ainur with an Eruhín Fána, were able to interbreed while no known mixed descendants of Elves, Halflings, Men or Orcs and Dwarves, Ents or Giants were known. The Origin of the Half-Trolls remains unsolved since it is not known if the people called Half-Trolls by the Gondorians really were halfbreeds of Trolls and men or just a mannish people with an uncommon and exotic appearance.Also Professor Tolkien seems to have been unsure if Orcs and Trolls could interbreed, or at last made contradicting statements.
Often however the term race was used not in sense of biological race but synonymous with People, Folk, Culture or even Tribe.
Roleplaying[]
In Roleplaying often Elves, Men, Halflings, Dwarves, Orcs, Trolls, Giants and Ents are treatened as distinct races.MERP, MECCG, MEPBM and LOTRO consider the above mentioned Half-Trolls true (though magically created) Hybrids of Trolls and men and not just a pejorative name for strange men of an unknown type and particular or foreign look. Also MERP added a race of Human-Dwarf-descendants known as the Umli and frequently mentions crossbreeds of Trolls and Orcs and even Trolls, Ents and Giants.