Holmes doesn't say anything about their origin or their appearance, only that they are organized by uncooperative tribes or nations.
|Orc waits to be classified. Pencil, 2011.|
Are they just big goblins? As I see it, goblins are fairies, manifestations of darkness perhaps, but essentially of the earth and not inimical to it. Orcs, on the other hand, enjoy trampling pretty flowers, building fiendish machines, and generally making a slag-heap of things. They have un-nature in them. So they're definitely not fey creatures.
Where do we usually find them? As minions of a BBEG or else as AWOL troops of same. Where do they come from? Are they just another mundane humanoid race, which is what they were watered down to by the 3e era? I've never been happy with the idea of orc families, with little orc babies running around. Orcs shouldn't have dependents cramping their style. They have a certain standard to maintain.
My answer is that they are created (or summoned) servants of evil. In my campaign, orcs are the foot soldiers of the devil, brought to earth by evil wizards' summon orc spells to serve in their usual roles as minions, guards, and troops. And they're pretty bad-ass. I see them as rawhide-tough, brutal fighters who might live centuries on earth once they're summoned, long outliving their original masters. And yeah, maybe they take earthly mates but they don't hang around to drive their half-orc spawn to archery practice.
Here's my working description of them for my blue box set:
An infernal soldier who is summoned by magic to serve its master.
Orcs are generally human in shape, short and broad, powerfully muscled, with long arms, short legs, pointy ears, bulbous eyes, and sharp teeth. Their skin is a livid red color. They carry heavy curved swords, long-bladed spears, and short, powerful curved bows. They are brutally disciplined soldiers, strong and tireless, and follow orders faithfully, but not mindlessly.
Orcs are native to the infernal lands and only appear on earth after having been summoned by magic. They most often encountered as the soldiers or sentries of malicious wizards. See the spell Summon Orc for details.