If you've seen Lord of the Rings, or read any other fantasy story in which orcs are portrayed, you probably think orcs are hideous humanoid monsters charging mindlessly forward to slaughter helpless innocents. Azerothian orcs are significantly different, however, with a shamanistic culture that prides honor above all other virtues.
But unless you've played World of Warcraft or Warcraft 3, you probably wouldn't know that. The orcs of Warcraft 1 and 2 were pretty squarely in "bad guy" territory, and it is only with the story of Thrall's rise to power and return to shamanism that we find out what the orcs' true history is.
Ironically, the story of the orcs is a bit like that of the horrors of modern Nazis and the lore of the ancient Jews mixed together. Imagine that the vast majority of your species came under the sway of a terrible and evil leader, utterly determined to commit genocide against your peaceful neighbors. After carrying out this deplorable task, your people sought a new enemy, and found a new world to destroy. In the midst of this conquest, however, your people's political leadership failed, the way back home was cut off, and you all ended up as slaves in exile, lethargic and utterly without hope. Suddenly, a hero appeared to unite your people, overcome your former masters, restore your ancient faith, reclaim your dignity, and establish a new homeland.
What follows is a brief account of the events most orcs know about or lived through, and a glimpse of the effects they would have had on your character.
The Final Solution
The opening of the Dark Portal and entry into Azeroth is even more of a defining moment for the orcs than for the humans. But for older orcs (like Drek'thar), the fall of their people began about 45 years earlier, under the respected but ill-fated leader Ner'zhul. Kil'jaeden deceived him into believing that the draenei were plotting to destroy the orcs. By the time Ner'zhul realized his folly, it was too late -- Kil'jaeden made Ner'zhul's apprentice Gul'dan into the new leader and stripped Ner'zhul of his power. The orcs were already set in motion along a path of doom, and the extermination of the draenei was already underway.
If your orc character about 70 years old, he or she would have been born at about the time Kil'jaeden started to corrupt the orcs, growing up during the greatest changes in the history of their race, and witnessing their trainformation from a spiritual race into a genocidal maniacs. Such an elderly character might have early memories about about how Ner'zhul was deposed and Gul'dan came to power by uniting the clans into a single Horde and guiding his puppet Blackhand to become the first Warchief. He or she would also remember the number of shamans declining as Gul'dan taught warlock magic to more and more orcs, as well as the defining moment when Gul'dan invited all the orcs to drink the blood of Mannoroth, bringing all those who drank under control of the Burning Legion. Orgrim Doomhammer refused to drink, and Durotan even forbade the entire Frostwolf clan from drinking, which brought down great suspicion upon them.
Exile
An orc born closer to the opening of the Dark Portal, 25 years prior to the setting of World of Warcraft, would have been raised in this bloodthirsty environment and would have been given the blood of Mannoroth as soon as possible -- unless he or she was a Frostwolf. The Frostwolves were exiled to the Alterac Mountains as soon as the Horde entered Azeroth. Here was the last remaining refuge for orcish shamanism, as Drek'thar finally turned away from the path of the warlock and regained the favor of the spirits. Thrall was also born here, around the same time that Orgrim Doomhammer killed Blackhand and made himself the new Warchief. When Durotan was killed, and his son thought lost, Drek'thar kept the Frostwolves going until Thrall finally returned many years later and brought renewal not only to the Frostwolves, but to the entire orcish race. If your character is a Frostwolf, he would be extremely proud of his leader and his people about now, having gone through a great deal of suffering in order to reach this point in history.
The rest of the orcs, however, did not fare so well. Things seemed to be going well after the First War, when the orcs headed north to finish off the humans in Lordaeron. During a time when Gul'dan was weak, Orgrim Doomhammer killed Blackhand and took over leadership of the Horde, forcing Gul'dan to submit to him. Later on, however, Gul'dan betrayed Orgrim and took off with a third of the Horde army in search of the Tomb of Sargeras. This left the door open for the newly formed Alliance to breach the ranks of the Horde plowing all the way back to the Dark Portal and destroying it. The humans reduced these orcs to slavery and kept them in internment camps, and, thus denied more access to the blood of Mannoroth, the orcs went into severe withdrawal and lethargy. Only Grom Hellscream's Warsong clan managed to remain free, hiding out in the wilds of Lordaeron, fighting the weakening listlessness of Mannoroth's blood curse until he met up with Thrall about 15 years after the end of the second war.
Let my people go
Perhaps more than any other race, the majority of the orcs can say that they shared in common most of the important experiences of the Third War (excepting of course, those elements we talked about last time involving humans and the Scourge). Since the end of that story, most of the orcs would consider Orgrimmar their new home -- but it is very important to remember that none of them could possibly have been born there, since Thrall founded new orcish nation of Durotar after defeating the Burning Legion. Older orcs would have been born in one of the regions of Draenor (which were very different prior to its becoming "Outland"), while younger orcs would have either been born somewhere in the conquered lands of Southeastern Azeroth (perhaps even the ruined Stormwind itself), or in one of the internment camps that orcs were later removed to.
Durotar has very much of a "Promised Land" feeling about it to those who believe in Thrall and in his cause -- but not all orcs are Thrall's devoted disciples. Especially if you are playing a warlock, you may live on the fringes of modern orcish society, continuing activities the Warchief has tried to forbid, including dark magic, slavery, and perhaps a lingering bloodthirstiness.
One final note: orcs sometimes have an undeserved and incorrect reputation for being incredibly stupid. This is probably caused by the image of orcish peons walking around doing menial tasks and speaking with poor grammar (i.e. "Me not that kind of orc!"). Remember: the peons who speak that way are a kind of "untouchable" class within orcish society, who are reduced to menial labor because of limited capacity, mental or otherwise. Thrall believes that they have the same potential as all the other orcs and is trying to improve their condition, but it is still unlikely that such an orc could rise to be a great adventurer just yet.
For further reading about the orcs, check out WoWWiki's mostly good (but sometimes inconsistent) information about the orcs and their various heroes, as well as the much-simplified Dramatis-Personae page on creating an orc of your very own, and if you are a member of Dramatis-Personae's roleplaying forums (as I suggest you should be), you can even read this thoroughly researched outline of various orcish clans and choose which clan your orc belongs to (or you can read about the clans on WoWWiki as well)! You may also find this updated timeline at WoWWiki useful as a reference.