Human is a creature type.
Humans have no mechanical ties, and yet they still have some unique traits. For one, though they are a majorly supported creature type, there were, up until recently, no cards that explicitly help or care about humans. The Innistrad card Hamlet Capitan gives humans +1/+1 when it attacks or blocks. Most support humans will help other creature types heavily populated by humans (such as soldier or barbarian), but it is rare for a card to specifically affect other humans. It is also relevant to note that both tribal blocks, Onslaught block and Lorwyn block, kept humans out of tribal support (and Lorwyn totally excluded humans from the entire block).
Humans are also readily included in all colors for all roles. They are essentially the hole-filler creatures who can be anything, but are generally one-man armies.