What is a gear driven sprinkler head

A gear driven head is different from the old "impact" sprinkler heads that were installed for years.  Impact heads are rarely available anymore and for good reason.  

A gear driven head is a closed sprinkler head that rotates using water power within a gear mechanism enclosed in the sprinkler head.  These heads have been in use for a long time now and are the only heads installed.

Gear heads are used in areas that require watering for areas between 20-40 feet.