This article or section contains obsolete information, but exists here for historical purposes.
Clairvoyance was a 'targeted' summoner spell. It was available on Summoner's Rift, The Twisted Treeline and The Crystal Scar, but got removed from Summoner's Rift and The Twisted Treeline at some point. becoming exclusive to The Crystal Scar and the Dominion game mode, which got removed later as well. It was mainly used for scouting, aiming skills, avoiding enemy ganks and setting up ganks of your own. It could not detect invisible units, but it could see into brush. Unlike wards, Clairvoyance needed not be "placed" into a specific patch of brush, but instead it revealed all brush within its range.
Clairvoyance was generally taken by a support champion such as Sona or Soraka so that carries may choose more self-serving Summoner Spells.
Clairvoyance was primarily used to check the enemy jungler's progress, and prevent ganks from the jungler or laners that are "MIA".
If you know the enemy jungler is invading your jungle, cast Clairvoyance at their probable location, hopefully catching them off guard and granting your team a kill.
You can use Clairvoyance during a team fight near brush to prevent your team from losing line of sight.
Champions that have ability with high or global range can use Clairvoyance to be sure that the ability will hit its targets.