Explanation

yes please ! I wanna know as well.
I'm new to this site so get easily confused what what is (⊙…⊙ )

Shounen- action like mha manga
shoujo- romance
slice of life- every day things idk
harem- a a lot of love intrest on the mc like 5 guys liking the girl/boy

Doujinshi- hmmm idk how to explane it but itss somthing like levixeren but w smut

Seinen is like men intrest and joesi is women. here is the meaninng from google -Seinen = what interests men and grown ups in general. More serious and complex themes, sometimes politics, longer dialogues, gruesome and graphical action, etc. Josei = what interests women

Shounen is made for boys
Shoujo is amde for girls
Mecha are robots
Echhi is boardline porn
Slice of life is a evening cup of tea, no drama
Harem i got hoes

idk what meecha and ecchi means tho, sorrry

shounen is targeted towards teenage male audience.

seinen is toward older men or adult men.

shoujo is towards teenage girls.

josei is towards older women or girls who are already of legal age. (not too sure about this one)

ecchi is like porn or sexual themes.

harem is where the mc has a lot of love/sexual interest around them.

doujinshi is basically like a fanfiction but in drawing form, it's based of another manga/webtoon.

slice of like is just a very simple narrative of a person's life, it often lacks plot and is just simply a person's daily life being narrated.

idk about mecha though, i haven't read that many in this genre. hopefully i helped u though! :DD

Shounen- Targeted toward boys but can be enjoyed by everyone. Typically contains action. (One Piece, Naruto, etc.)

Josei- Geared toward adult women or older teenage girls. Stories would typically contain more mature and realistic depictions of love rather than shoujos. (Karneval, A Silent Voice)

Seinen- Marketed toward adult young men. More mature than Shounens. (Tokyo Ghoul, DeadTube, etc)

Doujinshi- From what I understand it's usually some sort of fan made spin off of the series it is trying to replicate.

Shoujo- Intended for young girls to read. Mostly full of isekais etc. I wouldn't say that the love is unrealistic but it is much more immature in that aspect than that of Josei.

Slice of Life- Usually depicts what a normal life is for a character. Probably in a school setting and the characters are in highschool. Of course this isn't always but that's just the generalized version for it. (Horimiya, Pun Pun, etc)

Harem- This is...well, it is essentially what the definition of what a harem is. Generally it's a story about a male mc who gains the affection of multiple women. There are sometimes stories that center around a female mc too, those are called reverse harems.

I am not sure what Mecha is but someone will probably comment and fill you in on that.
If you need any clarification or a summary I will gladly do so. Hope this helped!

its like fanfiction but for manga w some additional smut to it

Can someone explain to me the difference between shounen and shounen ai

I know that shounen ai means like BL without smut/sex but what is the difference between the 2?

Actually
Shounen is a genre AND a demographic towards boys most of the times it just happens to be action but it can be whatever like romance or mystery

Josei is also a genre demographic focused for older women it has more mature themes and CAN be romance but a mix of other genres as well

Seinen is pretty much the same but targeted towards older men tending to be more violent SOMETIMES and it can also be ROMANCE

As for doujinshis I Think that they are fan made stories using characters from their favourite whatever (,movie, comic,cartoon etc.)correct me I'd I am wrong

Shoujo is the same as shounen but Is meant for girls and can be any genre nit just romance and doesn't have to contain romance

Okay this is where things start to get tricky for me but i think mecha is anything with mechanics being a huge part if what you are reading useally containing robots feel free to correct me if I am wrong

Ecchi is basically fanservice where they sexualise the female characters to please young make audiences as they are useally or mostly seen in shounen

Slice of slice is useally when you see you,'re characters going through ''normal'' everyday life doing for the most part doing nothing too crazy but other than that I'm unsure

And last but not least harems. Basically it's when the male main character is surrounded by females who harbour feeling toward him. You can also say the same bit with the roles reversed is a harem but it's useally called a reverse harem where multiple male characters have feelings for the female main character

Hope I helped in anyway I will look up definition that I am unsure about and will hive you a simplified answer

mecha is robot stuff, mostly futuristic and/or sci-fi

shounen is targeted towards teenage boys, this mostly contains action and things that r fighting.

shounen ai is non-explicit bl, it normally doesn't contain smut/sex. shounen-ai is like a sub-genre for romance but it's queer.

Shounen- all action, probably no romance or the romance is with a girl
Shounen ai- action and is a bl, sometimes they contain smut but it is usually minimal or not shown/implied

Mangaorian2000 has it right. But doujinshi are any independently published manga. It just so happens that most of them are fan comics, but you can read original works, too.

Thank you for the correction

Thank you everyonee for your help !!