Otherkin (sometimes called "otherkind") are people who identify in some nonphysical way as nonhuman or not fully human, more specifically beings who do not currently exist on earth (though it is often used as an umbrella term). This identity is often considered to be involuntary.
What you identify as is called a kintype. Various kintypes include but are not limited to elves, demons, fae, dragons, angels, and many others. Someone may explain their experiences in various ways including through reincarnation, walk-in events, brain wiring, an unconscious coping mechanism, and many more.
There is overlap between Otherkin and Therians due to the fact that the two concepts are similar. Some may consider therians to be a sub type of Otherkin as they use otherkin as an umbrella term for nonhuman identities, but others consider them to be separate entirely. Someone who identifies as a wolf might call themselves a wolf therian, wolfkin or both, and sometimes wolfkin is used to refer to wolves that have traits not found with Earth's wolves (wings, unnatural colors, and so on). Someone can also be both otherkin and therian, for example; a demonkin could also be wolf therian through having multiple kintypes.
Otherkin are often referred to as 'Kin for short. A kintype being the 'kin identifies as. As stated above being otherkin is very personal and beliefs of cause and experience vary from one individual to another. Some may believe their soul is a mixture of more than one species (hybridkin) and others may have more than one kintype (polykin). The otherkin community is a very diverse community that has many different beliefs and opinions.