Call Target is a keybind ability (default: + ) that allows you to "mark" a target, monster or player, which creates a large red bull's-eye that hovers over the head of the enemy.
If an enemy that is targeted uses an invisibility skill, such as Shadow Refuge the target will be removed and have to be replaced.
This allows you to quickly identify and re-acquire this target in cases where there are numerous enemies that require abilities applied to them, or in cases where your target is forcibly lost -- such as when a Mesmer uses certain abilities. Calling a target also allows for teams to more easily focus their attention on a particular target. This is especially useful against Mesmers because, once marked, it allows you to easily identify the "real" mesmer among otherwise identical clones.
The Call Target mark is visible to your entire group, and only one target can be called at a time. This means that if one player calls a new target, the previously called target will no longer be marked. All players in the group can press "T" to switch their target to the called/marked target.
Call Target is not visible to players who are not in your party.
An update in the second half of 2013 changed the functionality of target calling to allow players that were not in a party to call targets. Previously, players would need to invite an offline player to their party so they could use the target function when solo.