Celestial Spirits are Magical beings from their own separate universe, the Celestial Spirit World. Some Mages, called Celestial Spirit Mages due to their specialization in Celestial Spirit Magic, are able to summon Spirits by opening their Gates via the use of Celestial Spirit Gate Keys. These Keys are separated into two classes: the more common Silver Keys and the extremely rare Gold Keys. Keys for Celestial Spirits are counted in Units (collectively without regard of Gold or Silver), and a Celestial Spirit Mage is noted by how many Units they have obtained. All Celestial Spirits are each named after a different astronomical constellation. Out of the twelve Gold Keys, ten are owned by Lucy Heartfilia and the remaining two are owned by Yukino Aguria, who also holds the key to the "13th Gate" of the Zodiac.
When a Celestial Spirit Mage receives a key and opens its Celestial Gate for the first time, they have to set up a contract with the respective Spirit. This contract consists of asking the Spirit which days it can be summoned by the Mage. This simple agreement forms a dedicated and serious bond between the Spirit and summoner. However, under certain circumstances, i.e. during battle, a contract can be asked to be completed later.
Contracts can be broken if the Mage is arrested, releases the Spirit by him/herself, or dies. When a Celestial Spirit Mage summons a Spirit, it appears directly next to them, as that's where the Gates to the Spirit World have been opened. It's impossible to summon Spirits elsewhere. The Spirits must also abide by certain rules enforced by the Celestial Spirit King. The only rule that has been introduced in Fairy Tail so far is that a Spirit may not directly or indirectly kill its summoner under any circumstances. When the Gates are closed, it is required from both the summoner and the Spirit to agree upon the Gate's closure. However, Spirits can also be "forced closed" in battle, which means they can be forced back into the Celestial World by the summoner's will, though not every Celestial Spirit Mage is capable of doing that.
Celestial Spirits also have different categories of sorts, which fall under the basic summoning conditions from their owner. For example, Loke summons himself most of the time without Lucy doing so herself, Virgo also has summoned herself once, as have Gemini and Horologium and if trust between the Celestial Spirit and the owner is not an issue, contracts can be temporarily dissolved. It also seems that if the Celestial Spirit is strong enough, they can use their own Magic to stay in the Human World, when or if the contract is not in action. The strength of the Celestial Spirits is affected by the Magic Power of their summoner, as Lucy once said that her spirits could become even stronger than Natsu if she trained more.