Ghost is one of the more powerful Tier 1 perks in the original Call of Duty: Black Ops. The Ghost perk allows players to avoid radar detection while an enemy Spy Plane is in the air, giving it similar attributes to the UAV Jammer and Camouflage perks from other Call of Duty titles.
The Ghost perk also allows players to wear ghillie suits without using sniper rifles, granting them the added benefit of optical camouflage by making them more difficult for players to see, especially on certain maps.
The Pro version of Ghost allows players to avoid being targeted by automated killstreaks like Gunships, and prevents players from glowing white in the Infrared Scope. In addition to other benefits, the most powerful ability offered by Ghost Pro is for enemy players' crosshairs to not turn red upon targeting a player with the perk enabled.
In short, it's kind of a big deal.
Treyarch's David Vonderhaar has taken note of how imbalanced the perk can be and has stated on Twitter that the developers plan to make Ghost a high level perk instead of making it available to players from the get go. You'll have to earn your way to it this time around.
Been working on the level requirement progression all day. I'm thinking
#GhostPerk is going to require a high level. +
— David Vonderhaar (@DavidVonderhaar) September 4, 2012
Granted, players who want to use Ghost are going to use it as soon as it unlocks, but it'll give low level players a chance to build up their skills before engaging near-invisible targets later in the game's cycle.