In all seriousness, "Adult" and "Mature" have a lot to talk about. I will give you the long answer but there will be a table in the end in case you want it short.
Adult
By definition, it requires you to be adult (as defined by your country's laws) to read. Most often, it refers to the age of your physical body.
Mature
It is the maturity of the mind in contrast to the body.
To sum up,
Adult is more or less about your biological body (hormone, reproductive organs...) + some bollock requirement from the laws (Sex ED...).
Mature is about your mind, your thinking. In short, an
Adult body vs a
Mature mind.
Adult
Saying that, this genre focuses on pleasing your physical need with sub-tags and theme mostly involve sexual intercourse.
Mature
Focus on themes and storylines that are pleasing to a
Mature mind. Thing that are serious and usually not very enjoyable to immature mind.
For readers.
They help setting expectation. Thing audiences can expect from an
Adult work will be the introduction of sex and may be gore (i want to note that gore is not really an adult or a mature tag but it is considered adult/mature because of the law). Meanwhile,
Mature work are usually about complex things like grey morality, controversies about war and politics, philosophic discussion about aspects of life and nature, the grandeur of intelligence (strategy...) and wisdom etc.
For authors.
They help filtering the demographic of your readers.
You can expect that mostly legally adults will
read your
Adult fiction (I want to stress that i use
read as in "like to read"). However, that does not mean underages will not want to read your work. No one knows whether there are underages within your Adult work's readers or not.
For
Mature, most of the time it can be argued that only adults will
read your fic, but that isn't set on stone. Some people develop a taste for
Mature stories very young while some develop much later. The demographic in this genre is also varied.
| Adult | Mature |
Demographic filter | (18-45)+, may attract some younger age | (18-45)+
but may attract much more younger age |
Tags | Primarily sexual-related tags | May or may not have sex or anything sexually obscure |
Definition | Maturity of Body | Maturity of Mind (and, may be, Body) |
Fulfillment | Full fill the sexual need of the readers | Full fill the mental need of the readers |
Usage | Tell your audiences that you expect them to love some sex in the fic | Tell your audiences that you want them to think and comment like mature people |