Steven Universe

Steven Universe, or simply Steven, is an animated series on Cartoon Network. It is about a young boy called Steven Universe, who is half gem (an alien race who are made of gem crystals) and half human, and follows his story with the Crystal Gems, a rebel group against the Gem Homeworld.
Context.
Gem Society.
Gems are an all-female-appearing alien race made up of a gem crystal and a "hard light" form that makes up their personal appearance. Gem society is divided in such a way that each type of Gem, such as diamonds, pearls, and so on, have a role in society that they were made for.
Diamonds are the ruling class of the Gem Homeworld, served by servant Pearls. There are four known diamonds: Pink Diamond, Blue Diamond, Yellow Diamond and White Diamond. All other Gems are either assigned to a Diamond (apart from Pink Diamond, because she is not present on Homeworld), or unaligned. Unaligned gems are either rebels or corrupted (corruption is the corruption of a Gem's hard light form, forcing the Gem to take on the appearance of a monster).
Homeworld colonises other planets so that new Gems can be formed from the minerals found in the ground; this had a devastating impact when Earth was in the process of colonisation.
The Crystal Gems.
The Crystal Gems are a rebel group led by Rose Quartz . They previously fought Homeworld because Rose Quartz wanted to protect the life on Earth. After the Diamonds (Yellow, Blue, and White) forced the end of the war with the corruption of all Gems on Earth (aside from the main cast of Crystal Gems, because of Rose's shield), the rebel group went into hiding, although since the birth of Steven, they have had more contact with Homeworld. (Rose gave up her physical form to make Steven, so she was never really in the show other than memories from the Crystal Gems and Greg ( Steven's dad) 
The Crystal Gems are currently composed of Steven, Connie, Pearl, Garnet, Amethyst, Lapis Lazuli, Peridot and occasionally Bismuth at times.
Fusion.
Fusion is the process in which the hard light forms of two gems are mixed together by choice to create a new form, and a new personality of the Gem. It can only be achieved by perfect synchronisation of two Gems. Steven is an exception to this rule - because he possesses Pink Diamond's Gem in his body, he is capable of fusing with both humans and Gems, for example, with Connie to form Stevonnie.
Fusion between different Gem classes is strongly discouraged on the Gem Homeworld.