Regarding this, what is shoujo and shounen?
Shounen, first of, is a japanese for a teenage boy. Shounen can be focused on boys but there are plenty of girls that enjoy just as much as boys. Shounen is just the name of the comic genre. Shoujo basically means girl in japanese. In manga and anime cases, this means girl manga.
Furthermore, what is the difference between shounen and seinen? ? means boy or young boy. Seinen/?? means roughly young adult or adolescent. Seinen manga are generally marketed toward 'young adults' which usually means high school age and up. Seinen manga generally have no furigana attached to the kanji and themes are much more diverse than shounen (or shoujo) manga.
Subsequently, one may also ask, what is the shoujo genre?
???, shōjo manga) is manga aimed at a teenage female target-demographic readership. The name romanizes the Japanese ?? (shōjo), literally "young woman". Strictly speaking, however, shōjo manga does not comprise a style or genre, but rather indicates a target demographic.
What's the difference between Yuri and shoujo ai?
Yuri is generally a form of fanspeak amongst fans, but its usage by authors and publishers has increased since 2005. In Japan, the term shōjo-ai (???, lit. girl love) is not used with this meaning, and instead tends to denote pedophilia (actual or perceived), with a similar meaning to the term lolicon (Lolita complex).