Orsimer are orcs. There is no distinction between the two in any TES book or game.
http://www.uesp.net/wiki/Lore:Orc
The Orcs were supposedly created when the Daedric Prince Boethiah defeated the Aldmeri god Trinimac, transforming him into Malacath and his faithful, the Orsimer, into Orcs. When the transformation occurred is unclear, but Orcs have been inhabiting the Iliac Bay region since the early years of the First Era. Some reports say Orcs were in Morrowind acting as raiders and mercenaries as early as 1E 700. It is known that the Aldmer had already colonized the mainland of High Rock during the Merethic Era before Orcs emerged. They were viewed as a constant threat to the other races, especially after the Ra Gada drove many Orcs out of Hammerfell and greatly strengthened Orsinium. Several kingdoms banded together in High Rock and destroyed the first Orc kingdom in 1E 980. Without a home, prejudice against them was even greater.
Meaning they are, indeed, elves; or at least, were in the beginning. The quest "Discerning the Transmundane" proves that the Orcs truly are elven.
Spoiler Hermaeus Mora's follower even says "the blood of all elves" (paraphrased). Anyone who knows the lore behind the Princes know that Hermaeus Mora's sphere is all knowledge; forbidden or not, known or unknown, Mora knows it.
Just because they don't look like elves don't mean they aren't; "Mer" literally means "Elf" and the Orcs are Orsimer.
(EDIT: Grammar)