In The Book of Lost Tales, it is said that Orcs were "bred from the heats and slimes of the earth" through the sorcery of Morgoth. Again, Tolkien later changed this, as Morgoth could not create life on his own. This led to the most popular theory that Orcs were created from corrupted Elves.
The first Dark Lord Melkor took hostage some Elves from Cuiviénen. He tortured them, beat them, and broke their bodies into the first deformed and twisted beings known as Orcs.
The orcs appear (especially in The Lord of the Rings) as a brutish, aggressive, ugly, and malevolent race of monsters, contrasting with the benevolent Elves. They are a corrupted race of elves, either bred that way by Morgoth, or turned savage in that manner, according to the Silmarillion.
Tolkien does not say how Orcs were bred or how Uruks were differentiated from other breeding populations of Orcs. However, in The Silmarillion the narrative suggests the Elven loremasters speculated that Melkor (the first Dark Lord) may have bred the Orcs from Elves whom he captured in eastern Middle-earth.
Uruk-hai were later bred by the wizard Saruman the White late in the Third Age, by his dark arts in the pits of Isengard. The Uruks in the service of Isengard used the symbol of the white Hand of Saruman, featured on their banners, helmets, and faces.
The Nazgûl, as servants of Sauron, feared the power of Ulmo, and believed that some of his power might still flow through the waters of Middle-earth, hence why they avoided it as much as they could.
Aragorn is not half Elf, although he is a descendant of Elros, who is half Elf (and the brother of Elrond, the half-Elf who raised him), which explains why Aragorn's life span is unusually long.
Literally murdering Tolkien's work with this one. Orcs are meant to represent how British soldiers were supposed to see German and Austro-Hungarian. They're born as adults in scum and mud and bred only to kill, they don't have a sex because they don't sexually reproduce.
It is unknown if the Orcs were immortal like the Elves. There is, in any case, a hint for a long lifespan in the story of two of the most famous Orc-chieftains: Azog and Bolg. Bolg, being the son of Azog, was the chieftain of the Orcs who attacked Erebor in the Battle of Five Armies in T.A. 2941.
Due to his corrupting influence, he swayed many of the Maiar, lesser divine beings including Sauron himself, to his cause. After the Earth was created, Melkor was the first to learn of the awakening of the Elves, and before the other Valar can find them, he kidnaps and cruelly tortures some, twisting them into orcs.
As Galadriel explains, in the First Age, the Dark Lord Morgoth captured and tortured elves, turning them into twisted, ruined creatures that would later become the orcs as we know them.
Orcs were the primary soldiers of the Dark Lords' armies and the most common of their servants. They were conceived of by Morgoth during the Years of the Trees, serving him and later his successor, Sauron, in their quest to dominate Middle-earth.
Some went mad and killed themselves, others fled and likely formed their own communities. Last time Sauron was defeated they still held together bands that were strong enough to kill Isildur, so they can operate well enough independently.
In The Book of Lost Tales, it is said that Orcs were "bred from the heats and slimes of the earth" through the sorcery of Morgoth. Again, Tolkien later changed this, as Morgoth could not create life on his own. This led to the most popular theory that Orcs were created from corrupted Elves.
In the Silvan Elvish language "Adar" is Sindarin for "father." Adar is the father of the Orcs, in a sense. Tolkien's novels give a few different potential origin stories for the Orcs themselves, but The Rings of Power puts the creation of these creatures in a new context.
Saruman : Do you know how the Orcs first came into being? They were elves once, taken by the dark powers, tortured and mutilated. A ruined and terrible form of life.
Orcs have elven blood, but are usually considered to be both Beastfolk and Goblin-ken.
For orcs, the age at which they're officially "adult" comes a little later than humans. Orcs reach maturity at about age 18-20. Middle age is around age 40 or so, old age at 65, venerable age at 80, and they rarely live beyond 100 years of age.
In early editions of The Guide to Middle Earth, Sauron is described as "probably of the Eldar elves".
As such, the Orcs were literally born in the dark, fighting and thriving in the black of night. When the sun rose over Middle-earth for the first time years after their initial creation, the light traumatized the Orcs, burning and blinding them after so many years shrouded in darkness.
Orcs and humans can interbreed, and in this union create half-orcs. Humans and orcs have been enemies for decades and half-orcs represent something both races prefer not to think about.
Orcs and humans have both held the Armlet of Strength and half-orcs are capable of interbreeding with both orcs and humans and continuing to produce fertile children. Interestingly, half-orcs like half-elves can also breed with other half-orcs and have created a fairly stable half-orcrace.
Legolas is a Sindarin Elf from the Woodland Realm of Northern Mirkwood. His father, Thranduil, is the King of the Silvan Elves living in that realm, making Legolas the Prince of Mirkwood.
After he was attacked by the orcs, they transported his lifeless body to Mordor at Sauron's behest. Sauron revived Isildur with one of the nine rings, and then tortured him until his spirit was broken and he became a Nazgûl.
Legolas is an elf, so he is immortal. However, he is only immortal in the sense that he will never die of old age or sickness. He can die in battle.