The Infinity Stones are six powerful relics within the Marvel cinematic universe.
Before the universe began, the stones existed as six singularities that were compressed down into the form ingots with the formation of the universe. They came to be known as relics of immense power and were used by several races to advance their respective societies and wage war.
By the modern age the stones were scattered across the universe; lost or haven fallen into legend. Few knew of their existence and fewer still sought their powers; the search made difficult by the fact the stones has fallen dormant making them difficult to track.
In Avengers Assemble the Space Stone; now known as the Tesseract, was awoken from its dormant state by the experiments of SHIELD. This draws the attention of Thanos who gives Loki control of the Chituari to take Earth in return for the stone being given to him. The invasion is thwarted by the Avengers and the Tessaract is entrusted to Asguard for safe keeping. Unknown at the time the Mind Stone arrived on Earth; encased within the scepter Loki used to command the alien army and was taken into the custody of Shield who were unaware of its true significance.
In Thor the Dark World the Aether was rediscovered and attatched itself to Jane Foster when she inadvertently found its resting place during the convergence. In the end the Aether was entrusted to the Collector due to the danger posed of keeping two stones close together.
The Power Stone was central to the plot of Guardians of the Galaxy and after the defeat of the Kree zealot Ronan was entrusted to the Nova Corps for safety.
In Age of Ultron, experiments on the Mind Stone lead to the creation of the malignant artificial inteligience Ultron who plance genocide against humanity. In the course of the battle the Avengers are joined by the Maximof twins; siblings granted enhanced powers after surviving HYDRA experiments with the stone.
Each stone holds immense power and dominece of an element of the universe; Time, Reality, Soul, Mind, Space and Power. Because of their immense power only beings of exceptional strength can safely wield an Infinity Stone. If a lesser being were to try they would be vaporized in seconds. It is possible for a group of lesser beings to wield a stone together; sharing the power between themselves, but only for a short amount of time.
The stones also appear to possess a vague lever of sentience; reacting when in close proximity to one another and will at times act on the subconscious desire of the wielder. When holding the Power Stone, Peter Quill was given a vision of his dying mother; encouraging him to take her hand, an act he had regretted being unable to perform in the past. A scan of the Mind Stone in Age of Ultron revealed its energy resembeled a brain pattern.
When left unused the stones enter a dormant state; lessening the power but making them easier to be manipulated and studied by others. However, when one stone comes out of its dormant state it becomes more powerful. The awakening of one stone also leads to the awakening of the others.
When in close proximity to one another the stones energy will react and intermingle with one another. The full scale of this unkown but is claimed keeping the stones together for an exted period is dangerous. When the Mind stone came in close proximity to the Space stone, the earthbound stone began releasing large arks of energy.
It is possible to contain a stone inside another object for transport and allow a person to wield it with out needing to touch it. The Power stone was sealed with an Orb in such a manner while the Mind stone was contained within the Chitari scepter.