Headstand
The head stand is a pose that is an inversion posture of standing head up. The technique is used in different settings, like yoga, breakdancing and beginner gymnastics.
In Yoga
The head stand is called Sirsasana or Sirshasana in Hatha yoga.
In Breakdance
Breakdancing features a number of moves that feature balancing on one's head. Headstands can lead into a head spin or act as a stand alone freeze move. Different from the yoga style, headstands ideally culminate with taking the hands off the floor and entirely supporting oneself with the head.
In Gymnastics
The headstand, although not an "official skill" (FIG code of points) is used in gymnastics as a precurser to many skills such as the handstand, headspring, ect. A gymnastics headstand can be done in any way; head only, head plus forearms, or head plus hands. Although it is often entered via a press, it can also be jumped into, although this is quite dangerous.
External links
To perform a good headstand you should make sure that you put your two hands and your head in a triangle to keep good balance, therefore being able to hold a longer headstand.