Eavesdrop is a Call of Duty 4: Modern Warfare Tier 3 perk that gives a player the ability to listen to enemy voice chat. Eavesdrop will only work when a player is in close proximity to enemy players. A player will be able to hear both allied and enemy voice chat simultaneously.
The most useful way to use Eavesdrop is in Search and Destroy, where a player can not only hear enemies in close proximity but can also hear members of the opposite team who have been killed and are waiting for the next round, despite not being able to hear his own killed team mates. In this situation, a dead team mate can manipulate enemies into revealing the location of the players still alive in the round, which is especially effective in a 1v1 situation if the Eavesdrop player is told where the enemy is planting.
It is also ineffective because players will often be in parties, not have headsets, or at least not talk about anything which would benefit the user, such as where other players are going, tactics etc.
On the PC, it is nearly useless, as many players use different programs on the side to communicate or text chat, which the Eavesdrop ability of course does not pick up.
Eavesdrop seems to be more useful in Hardcore, because when throwing a grenade or firing into a crowded area, a player with a microphone is likely to inform his teammates before performing the action, and thus notifying any nearby player with Eavesdrop.
Another possible use of it would be to mute all of one's teammates and just use the perk; upon hearing a voice, the player will know an enemy is near. This can be very helpful if an enemy is very chatty or has a broken mic (constantly providing a buzzing noise to guide the player). Although more effective than using it for hearing enemy tactics, it is somewhat limited because any person on the player's team will need to be muted to make sure that the voice is that of an enemy, depriving one of their helpful game info, if any (this also means that the user cannot use Eavesdrop in this way if one is in an Xbox Party or playing with friends). Furthermore, anyone who has just joined the game must be muted.
Eavesdrop is a perk that appears in Call of Duty: Ghosts as an awareness perk. However, it is unrelated to the perk with the same name in Modern Warfare. Eavesdrop allows players to hear enemies louder, similar to SitRep Pro and Awareness from previous games.