Though thorough, Miki's summary has many deprecated rules mentioned in it.
Field Hockey (grass hockey) is governed internationally by the Federation Internationale de Hockey, or simply FIH. This organization is responsible for changes to the rules accepted at the World Cup as well as the Olympics.
Wood sticks (usually Indian Mahoghany) are very popular, however composite sticks made of Fibreglass, Kevlar, and Carbon (Graphite) are now at least as widely used as wooden ones.
The rules can very simply be boiled down to a few sentences; Players try to score the ball by manipulating it with the flat side of their stick. No player may obstruct the fair play of another player by any means, and no player may create danger with ball, stick, or body.
However, the rules become more complex when one attempts to define words like danger and obstruction. Thus, Field Hockey is known as highly dependent on the discretion of the umpires, of which there are 2.
The rules have been simplified in recent years. The "flat side of the stick" now includes both the leading and following edges. A raised ball is legal at all times so long as it does not create danger in the umpire's opinion. The striking circle is 16 yards in radius from the nearest edge of the goal, making it flat at the top. Halves are 35 minutes long with a 5-10 minute half time.
Hockey is known as a "Schoolgirl's game" primarily in the United States, and an Aussie or Indian will outright laugh in your face if you refer to it as such.
For more information, visit http://www.usfieldhockey.com .