The Ghost is a type of "being" able to be summoned by the summoning jutsu. The contract with a ghost is found at the Shrine of Hatred, where it is said that the blood of many people and waring clans has been soaked. If one is to write their name on the parchment found in the shrine, in their blood, they may be able to summon a Ghost. However, a Ghost can only be summoned if the summoner is filled with intense hate, and is summoned for a hateful and violent reason. Only once the summoner can feel nothing but hatred, may he summon the Ghost. If the Ghost is summoned in any other way, such as happiness, the Ghost will not be summoned, and will instead, take over the body of the summoner and go on a rampage. This however, can be conquered once the user has conquered hatred, and the ghost may be sealed inside the user.
A Ghost's appearence changes depending of the chakra type of the summoner. For example, if the summoner's chakra type is Earth, then the ghost's body will be made of Earth. But all ghosts have 2 hands, each with 3 claws, white slanted eyes, and an evil grin. A ghost is typically very large, easily the height of a 2 story house. Whatever their body is composed of can easily regenerate since they are made of pure elements. There are only two ways to harm a ghost. One way is by the chakra natures weakness (For example, Fire jutsu againts a Ghost of Wind), and the second is by complete destruction of their bodies with a powerful jutsu.
A ghost is made completely of fear and hatred, and is literally, a born manifestation of it. It has no feelings or ideals other then to hate and kill everything in its path, or when summoned, everything the summoner hates.
The abilities of a ghost change according the the chakra type of the summoner. For example, if the summoners chakra type is Wind, then the ghost will have Wind abilities. All ghosts are very strong, and powerful summons, able to single-handedly beat most Chunins. But it requires a large amount of chakra to summon one, so its use is typically restricted to Jounins and S-Rank Ninjas.
Here are a list of the different abilities per element:
Ghost of Wind- Can shoot concentrated wind blasts from its mouth
Ghost of Water- Can shoot highly presurized streams of water to cut through its enemies
Ghost of Fire- Can shoot continuous blasts of flames from its mouth
Ghost of Lightning- Can shoot lightning directly from its hands
Ghost of Earth- Can shoot small projectiles of Earth at a rapid rate