Humans are probably one of the most prevalent species in the multiverse. They are four-limbed bipedal primates of the mammallian class. They have a highly developed brain capable of abstract reasoning, language, and introspection. The only natural plane where humans are known not to exist at all is Lorwyn. In the Innistrad block humans are a supported tribe. 
Types of humans
In most planes, humans act, and have the same physical traits. Notable exceptions are:
The humans of Mirrodin adapted to the metal planet, as had all races imported to the plane by growing organic metal pieces on their bodies. Mirrodin tribes consisted of the Auriok of the Razor Fields, the Neurok of the shore of the Quicksilver Sea, the Moriok of the Mephidross, the Vulshok of the Oxidda Chain, and the Sylvok of the Tangle.
All species on Mirrodin have metallic growths and plates that permeate and cover their bodies because of the effects of certain substances on the plane that interacted with organic life. The humans all have such metallic growths, but color, material, coverage, and shape vary from tribe to tribe.
Bird maidens inhabit the myriad realms of Rabiah. There are an all-female tribe of winged humans who procreate with non-winged male humans; their offspring are always twins of the same gender, and if male, are without wings, and if female, with wings. After they give birth the bird maidens raise the daughters themselves, but leave their sons in a nearby village under the custody of people living there. Any Rabian village that finds abandoned twin boys will adopt and honour them, due to legends about the strength, luck, and magical talent of bird maidens' sons.
The bird maidens practice an art known as wind sculpting. They place diamond dust in sharply funnelled bone tubes and tie them to rock formations. The wind blows the dust against the stone, causing it to erode faster than usual. Eventually, over the course of several years or decades, a sculpture appears.
Two tribes of humans, the nomadic Vec and the settled Dal, inhabited the blasted plains of Rath. Along with the Kor, an allied race of nonhumans, the Rathi tribes were steadfast enemies of the plane's cruel evincars — save for the il, those of their numbers who joined the evincar's evil forces.
For more, see Rathi Tribes.
For unknown reasons, possibly to do with the higher standard of living, the humans of Ravnica have longer lifespans than humans on other planes. They are the only race that has members in large numbers in all of the plane's guilds.
Human as a creature type
The creature type "Human" was introduced in Mirrodin as a part of the race/class model.  The notion was first brought up in the public by Randy Buehler  and was met with negative reactions.  After much convincing a poll determined that 53.7% of the readers would accept the addition of humans to Magic at that time. As predicted, players eventually came to fully accept the change.
- Ambush-Party (Ambush Party)
- Ancestor (Auspicious Ancestor)
- Aladdin (Aladdin)
- Ali-Baba (Ali Baba)
- Ali-from-Cairo (Ali from Cairo)
- Autocrat, which shortly was a Minion, (Sengir Autocrat)
- Bandit (Scarwood Bandits)
- Being (Enchanted Being)
- Brother (Brothers of Fire)
- Cave-People (Cave People)
- Constable (An-Havva Constable)
- El-Hajjâj (El-Hajjâj)
- Farmer (Shu Farmer)
- Flying-Man (Flying Men)
- Folk of An-Havva (Folk of An-Havva)
- Harlequin (Chaos Harlequin)
- King (King Suleiman)
- Leper (Abu Ja'far)
- Maiden (Bird Maiden)
- Master (Master of the Hunt)
- Miracle-Worker (Miracle Worker)
- Mob (Angry Mob)
- Nekrataal (Nekrataal)
- Peacekeeper (Peacekeeper)
- People-of-the-Woods (People of the Woods)
- Ranger (Kelsinko Ranger)
- Scavenger and Scavenger-Folk (Scavenger Folk)
- Sindbad (Sindbad)
- Sister (Sisters of the Flame)
- Smith (Repentant Blacksmith) (other Smiths became Artificers as well)
- Townsfolk (Anarchist)
- Tracker (Tracker)
- Uncle-Istvan (Uncle Istvan)
Innistrad block even had a 'Human matters' theme.
1/1 Human Creature tokens (without a specific class) are created by the following cards:
- Mark Rosewater. (September 05, 2011.) "C'mon Innistrad, Part 1", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (October 04, 2004.) "Change For the Better", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Randy Buehler. (November 22, 2002.) "Debating Humanity", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Randy Buehler. (November 29, 2002.) "Human Nature", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Randy Buehler. (December 6, 2002.) "The Power of the Skull", Daily MTG, magicthegathering.com, Wizards of the Coast.