Orcs

Orcs are humanoid monsters that delight in violence and battle. They tend to be nomadic in nature, living in clans that move around, though they can sometimes form more permanent settlements.
The orcs are descendants of human and elf clans that worshipped a dark deity in exchange for power. Gradually, the dark deity warped and twisted their bodies and minds into the sadistic and cannibalistic creatures they are now.

Behaviour

There is a strong belief that orcs are a people that can be reasoned with, as long as one understands their culture where brute strength garners respect and social status. Anyone with such a mindset coming into contact with orcs will soon find themselves longing for a quick death, however!
Not only does an orc have no empathy or sympathy for other beings, they also crave violence and torment. They bear an innate need to not only cause pain to others, but also to feel physical pain themselves. It's not so much pleasure as an instinctual necessity. This violence is such a part of their very being that they will not only fight each other, but even torture one another as a means to pass the time. They might greet each other through physical assault, and every part of their culture involves pain or violence in some way.
Orcs have a strong sense of society, but no love or feeling of duty for each other. They have no sense of family, and children generally fend for themselves from very early on. It's against the very nature of an orc to give their life for another, but might certainly sacrifice others for themselves.
Orcs of the same type will generally not kill each other, but will generally prefer inflicting pain and death upon creatures similar to themselves. In fact, the more similar the victim is otherwise, the more the orc will enjoy killing it. For this reason they might lose themselves in a frenzy when fighting humans and elves, while kobolds or animals are like boring oatmeal. Which is more than a metaphor, as orcs will eat anything they kill. Including their own, when it happens.
Different types of orcs coming into contact with each other will always end with either one or both sides completely annihilated.

Gray Orcs

As their name suggest, gray orcs have skin that range from light to dark gray, and round ears. Their hair is usually black in colour, and they tend to have a lot of facial and body hair. Their ancestors were humans, but they've lost the individuality and adaptability humans are generally known for.

Green Orcs

Green orcs have skin that range from verdant to olive green, pointed ears, and usually black hair. They tend to be more lean than the gray orcs, and have very little body hair, and very rarely facial hair. Their ancestors were elves, though in their lust for power they gave up their longevity, gracefulness, and wisdom.

Other orcs

Other types of orcs do exist, though they're usually referred to by their own names. However, creatures such as the duergar and the derro were created the same way gray and green orcs were, but from other races. Given that they also have greater differences to regular orcs than the gray and green have between each other, both physically and mentally, they're seldom considered the same.