Okay to put it simply
Shonen = Young man
Shojo = Young woman
Seinen = Youth
What does this mean in terms of anime?
They’re demographics!
Examples of each would be
Shonen: My Hero Academia
Shojo: Vampire knight
Seinen: Tokyo Ghoul
None of the demographics in terms of tropes are that rigid anymore and it can be largely down to where things are published.
However, typically Shojo can have more of a focus on romance where shonen has more of a focus on battle which is due to the original stereotypes of those demographics. Seinen can have more mature plotlines due to being aimed at an older audience but as I say, this isn’t rigid. There are plenty of shonen that deal with mature themes.
Typically though, you’re more likely to find a tournament arc in shonen.
There are plenty of anime’s that are classed as certain demographics but aren’t exactly traditional and cause debate for if they should belong in those demographics. Tokyo Ghoul is a great example of that. But overall it doesn’t matter much I just think it’s neat.
@animenerds anything else you wanna add?