Orcs are easier to master than humans. So newbies should definitely play orcs (though many of them don't). More advanced players should play orcs if they are still developing their strategy. Even the top players should use orcs sometimes if they're in a more challenging match.
The Orcs have one advantage:Bloodlust. Bloodlust is the single most powerful spell in the game. It triples the damage of Ogres and doubles the damage of any other organic unit (sorry catapult fans) and only costs 50 mana. Ogre-mages are physically strongest unit in the game when bloodlusted. A Human player will have to be on his guard at all times to stop bloodlusted Ogres. Orc Catapults also fire faster than human Ballistas (sounds strange but it's true).
While Bloodlust is the best spell in the game, Orcs fall down in nearly all other areas. Their answer to the Mages are the Death Knights. Not only can the Death Knights be destroyed completely by Human Excorcism, but their spells aren't all that great anyway. The only spells Death Knights have that are even worth considering are Death and Decay (which humans have blizzard to match), Haste (which doesn't even work on Ogres), and Death Coil (which costs a lot). Sometimes Unholy Armor can be useful, but not often enough to justify the cost (2500 gold!) and it takes a unit's HP down by half. Whirlwind is completely useless, and the Skeletons that Raise Dead creates get killed too easily, do little damage, and can be excorcised just like their masters. Troll Berserkers also do less damage than their woodland enemies. If your opponent can counter your bloodlust, he's effectively shut down your entire strategy.
First of all, do the lumber bug (right click on wood before building a hall) and build a hall. When the hall is done that peon builds a farm. He should continue this throughout the game. As soon as your farm is done, begin training peons continuously. The first peon builds a barracks (near the entrance to your town if possible). The second peon builds a blacksmith. The third peon builds a lumber mill (with the bug). If the mine has 80,000 gold or less, expand to another gold mine (see The Art of Expanding, coming soon) and train peons there. This allows a continuous flow of peons during the stronghold upgrade. The next two peons go on gold, followed by one on lumber. Continue this pattern. When the smith is done, do the armor upgrade and have the peon harvest lumber. When the barracks is done, start training grunts and put the peon on lumber. The lumber mill peon also harvests lumber, but don't do anything with your mill just yet.
When you have about 20 or 30 peons and are halfway to level 5 guys (the last sword upgrade is around 50% done), upgrade to stronghold. Keep training peons at your second hall (if you have one). As soon as the stronghold is finished, build an ogre mound and a second barracks. Keep training grunts until you can make ogres, and then start making ogres constantly. You may want to consider a catapult or two, or maybe even a third town (if you have the gold). If you don't have a second town hall yet, now's the time to get one, along with a few more barracks. This will slow you down in getting to lust though.
When the mound and barracks are done and you have around 35 peons, go for fortress. Keep training ogres during this time. As soon as the fortress is done, build an altar of storms being repaired by as many guys as you can spare. Go for ogre-mages as soon as it's done, and then lust. It's your call from here on out. You should easily have 8 or 10 barracks to start pumping out the ogres and you should never attack with only 9 (18 or even 27 is MUCH better). Keep up the pressure and never let up!