Don't want to vote on something that I have no knowledge on…
You've received multiple answers concerning what's going on, so hopefully you understand that it's coming from a place of good intentions and that you don't mind one explanation post.
In the Grim Darkness of the 41st Millennium there is only war...
The Imperium of Man struggles to maintain a fading dominion over their reach on the galaxy by the sacrifice of their own humanity and literally trillions of human lives, they are opposed by the remnants of the Eldar, elf-like aliens that embody the adage "pride cometh before the fall", the Tau, the new kids on the block who like to show off their shiny toys and go around telling everyone else how much better they are, the Necrons, undead robots (don't think too hard about it), the Tyranids, an extra-galactic locust swarm that devours biomass, and Chaos, four Gods who basically embody doing all the evil for the lulz. And then there's the Orks.
The Orks have two histories. One is the lineage of a noble warrior race created by a powerful precursor race alongside the Eldar to combat the Necrons and their masters in what's known as the War in Heaven. The Krork, as they were called at the time, where intelligent and powerful, disciplined warriors designed to asexually reproduce via spores and built-in instructions in how to create technology up to capitol-grade spaceships. Alas, with the death of the Old Ones the remains of the Krork have degenerated over the hundreds of millennium into Da Orks.
The second history is that the Orks are born to fight. It is literally their purpose, their goal, their dream, and the only thing they care to know. They are a pure species that cares for only one thing. To fight. To kill. To wage War. Against everyone, against anything. But they're not evil, a subtle distinction that is lost by those who's homes are destroyed and families killed. They are simple and pure beings of destruction. Not evil and not good. They are a stellar case of blue and orange morality because their entire sense of identity from the individual to the largest Warband is built around war. And Orks have been waging war for a long time.
More in-depth, and a little less hammy, Orks are a fungoid species, born from spores excreted by the orkoid bodies as they sweat and bleed. These spores are hardy and capable of surviving in extreme temperatures in both directions, toxic and acidic environments, and even post-apoplectic landscapes and lead to the birth of squigs, gretchens, and Orks proper. Orks never stop growing, as they fight and survive, they will grow bigger, stronger, and tougher, but even the average 'normal' orc possesses skin like toughened leather and enough muscle to easily overpower all but the strongest human.
Technologically the Orks are capable of scavenging just about anything together, from firearms to airplanes to the afore mentioned spaceships, given enough raw materials and time. Though they look clunky and slapped-together, they certainly work to worrying results.
The culture of the Orks revolve around two absolutes, Gork and Mork, their twin gods, one brutal but cunning and the other cunning but brutal, and the WAAAAGGGGGHHHHHH, and no that is not a misspelling. A WAAGGHH is simultaneously a war cry, the name of a sufficient of group of Orks, and the name of the psychic field that all Orks are connected to. The WAAAAGGH field bolsters the strength and drive of the Orks connected to it, and it's suspected to even be a key component of making their 'technology' work. Few examples of this are as plain as the simple belief that painting something red makes it go faster, whether it's three times as fast is up to debate, but Ork's believe that Red makes it go faster and the WAGGH field makes it so.
The third primary aspect of Ork culture is their hierarchy. Might makes right, and if you're mighty, then you're probably right...ty, hm. The largest, strongest, and typically the oldest, Orks are the Warbosses, hulking masses of muscle and crude cybernetics that lead and bully everyone else. Other high-ranking Orks are Meks, Orks who are good with making technology, Painboyz who are the closest thing Orcs have to doctors, Weirdboyz who are psychic warriors who channel the WAAAGGGHH field into psychic attacks, and Nobs who are the elite and the equivalent to Majors or Captains in a traditional military hierarchy. Under all of them are Boyz, the typical Ork who can pick up some specialty from strapping rockets to their back to get to the fight faster to using conventional explosives to, but will otherwise focus on shooting and bashing and chopping. And under them are the gretchins, small subservient goblin-like creatures bullied by everyone else.
I hope you enjoyed this primer on Warhammer 40K Orks.