The Orks are a genetically engendered species created by an ancient race known as the old ones to help fight a war against the Necrons with the help of the Eldar.
They are a society based on the stronger and bigger the better, the most aggressive ones grow faster, they grow every time they kill something.
There are many ork based species that fight in an ork waaaaagh witch is a giant army formed when the biggest ork unites multiple clans and becomes a Warboss and then a warlord once they have taken a planet, the different species are Squigs, Grechins, Squigoths and Snotalings. Most of their ships are made from asteroids that they have built over. The Orks constantly produce spores that grow into weed-like plants under-ground and open with an eight foot tall ork and produce many more spoors when killed.
Their skin is thick an leathery because they were created to be very durable making it impossible to completely rid a planet of them without a planet-wide Exterminatus. They are also genetically engineered to never die so if an ork was chopped into pieces it could be stitched-up and be alive and working the Ork would be a bit disorientated but ready to fight.