Nen (Hatsu) is the manipulation of one's aura.

Basic Facts

En the spread of aura into the air to feel and locate other Nen users
Gyo ability to see the body aura of a Nen user or materials bearing Nen; this can be done by accumulating aura in the eyes
Ren is the ability to make the flow of aura around the body more extreme than usual
Ten the ability to keep the aura from escaping into the air; can make the body much stronger
Zetsu ability to stop the aura from flowing around the body; commonly used in spying

Nen Powers

can give force to objects
can spread out aura away from the body (flying aura, etc.)
can make the properties of aura change
can manipulate living beings
can create objects out of the aura
aura that particularly possesses attributes that is different from the first five categories