They are one of the most widespread races in the Warhammer 40,000 galaxy. They are extremely warlike and the whole society is geared towards constant warfare. Orks live for battle and will fight with anything that gets in their way, even other Orks. This constant need to fight is the expression of Ork culture, a fact that keeps the Orks from forming anything but temporary alliances with each other. If the Orks were ever to unite under one banner they could quickly sweep aside all opposition in the Galaxy.
Physiologically Orks are humanoid hulking brutes with huge slab like muscles and is capable of rippping apart a human with its bare hands. They have tough thick green skin which is highly resistant to pain. They can sustain the most horrific of injuries and stay in the fight.
In battle they favour crude large bore pistols called "Sluggaz" which are heavy enough to bludgen a man to death, larger "Shootaz" which are a larger longer range version of the "Slugga" with a higher rate of fire. Most Orks carry huge hand to hand weapons called "Choppaz" which like all ork technology can vary widely in apperance from large monomolecular edged cleavers to chainsaws with crackling electric fields. These weapons are by diversity of design, capable of cutting through the heaviest personal armour of the 41st Millennium.
In combat they can transform even the most common object into a lethal killing instrument. Orks aren't the brightest creatures in the galaxy, but they do possess a certain low cunning that can catch an overconfident commander off guard. In a word, the Orks are built for war.
See also: Orc