Orcs and human can interbreed and create the half-orc race. Since orcs and humans deeply hate each other some half-orcs will side with Horde, other with the Alliance. Most humans and orcs ignore half-orcs and think of this race as a unity. Half-orcs tend to have strange faces due to the bloodshed their families participate in. They have a more facial reaction than others. They are very brave and daring. Half-orcs demand attention and go through life or death adventurers with only a few ever succeeding.
Half-orcs due to their mixed breed nature have been considered outsiders for a long time. It seems unlikely that they will see things from the inside any time soon. Their unfortunate status of their race leaves them as their own race and forces them to join the Alliance (less likely) and the Horde (more likely).
Since a large number of half-orcs are members of the Horde, they study shamanistic practices, much like their orc relatives. They accept spirits much more than any other earthly race. If a half-orc is a member of the Alliance they may study the Holy Light and follow human philosophy.