Dr. Awkward said:
There's another option, and one that's more plausible than either of the above.
3. Orcs have a generally nasty culture based on strength and bravado. They raid because it's easy and profitable, in a manner something like vikings without boats. They hunt, herd, and fish, but almost never farm. They set up in a location, raid farmlands for foodstuffs and tools, and move along once the heat turns up or the supplies dry up. They trade when it is convenient to do so, and protect those who will trade with them, because sometimes you have an empty belly but a pile of gold and someone can bring food from over the mountain if they're paid well. They aren't ravening savages. They just have a brutal view of life and no respect for the property of non-orcs. However, they're not stupid and they have good relations with those they're not actually raiding, perhaps even including some governments that allow them safe passage between neighbouring lands in exchange for neutral relations.
For half-orcs, see my above.