Clairvoyance

Clairvoyance is a 'targeted' Summoner spell. It is mainly used for scouting, aiming skills, avoiding enemy ganks and setting up ganks of your own. It cannot detect invisible units, but it can see into brush. Unlike wards, Clairvoyance need not be "placed" into a specific patch of brush, but instead reveals all brush within its range.

Strategy
Clairvoyance should be picked by a champion who doesn't need another Summoner spell, usually a support like or.

Trivia

 * Summoner spell Clairvoyance and summoner spell Smite has the shortest cooldown of all the summoner spells.