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.
Clairvoyance is generally taken by a support champion such as Sona or Soraka so that carries may choose more self-serving Summoner Spells.
On Summoner's Rift, use Clairvoyance early on the enemy fountain (between 5 and 15s) to see enemies' starting items and lane setup.
Enemy champions' items will be updated on the summary window (Tab or O).
Clairvoyance is 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.
If Clairvoyance only extends over half of a brush patch, an enemy unit can remain unseen in the other half.
The "eye" that is placed at the Clairvoyanced area can be seen by the enemy, even if it is in the fog of war.
Clairvoyance will reset the markers for neutral monster camps on the minimap.
Casting Clairvoyance does not interrupt Recalls channel.
The "eye" (green by ally and red by enemy) of the effect indicates its duration.
Clairvoyance has the second shortest cooldown of all the summoner spells at 60 seconds, preceded by Smite at 40 seconds.