Half-orcs = Uruk'hai.
Uruk'hai means "Orc Folk", and are the result of breeding Orc and "Goblin-men".
All of the "...-'hai" groups (like Olag-hai [troll-folk] and Uruk-hai, etc) are characterized by being more cunning, faster, and stronger versions of their base self. It can be easily assumed that their crossbreeding with human or goblin-men imparted upon them some of the traits of that creature.
If you're interested, you could always go grab the Uruk'hai sheet off of D&DWiki. It isn't unbalanced.