My friend has sent me a work to critique which he hopes to make into a light novel. I've never heard of this type of story before and a quick Google didn't give me much context about how it differs from a traditional western young-adult novella?
I don't know about Sweet72's reference to Japanese literature or how that relates to English literary forms.
But in common American usage, a "novella" is a shorter work than a "novel". Some give formal definitions, like a novel is over 40,000 words while a novella is 20,000 to 40,000. But I think the real, practical definition is that a novel is a story that takes a full book, while a novella is a story of such a length that you would normally put 2 to 4 together to make a book.
Calling a story a "light novel" has nothing to do with length. Rather, that means that it is easy to read, fun and escapist, like an adventure story or a romance. This is as opposed to a "serious novel", which has deeper themes and embodies commentary on society or history or the nature of humanity. For example, "Atlas Shrugged" would be considered a serious novel, because the whole point of the book is to discuss the author's ideas about politics and economics. Any of the James Bond novels would be considered "light" because they are about adventure and romance and make no attempt at profound themes.
Of course any such distinction is vague and debatable. A book could include serious commentary about life while also being easy and fun to read. I've read plenty of books that are mostly adventure stories but that also include themes about the human condition. Personally I think a lot of the better science fiction falls into this category.
Either a light novel or a serious novel could be in pretty much any genre. You could write a light Western novel or a serious Western novel, etc.
As per Wikipedia's Light Novel article:
A novella is a short story and generally features fewer conflicts than a novel, yet more complicated ones than a short story.
The problem with trying to define what a Light Novel is has to do with a whole lot of cultural baggage that is hard to transmit over to a Western idea of Literature. So it seems to be a medium that is focused on word-count, like a novella, but that isn't a proper definition. Likewise it also seems to be attached to a certain light, popular, and easy-to-read style, but that also may not necessarily be the case. It seems to encompass genres such as slice-of-life, fantasy, sci-fi, mystery, or even a mix of all the above, but you can't exactly say that it sticks closely to any of these attributes.
It's definitely linked to popularity and the otaku fandom though, since within multiple series you can see a reliance of cliche and comedy tropes or action/fantasy tropes that you usually see in anime. Usually this means novella-length, genre-focused, cliche-focused, low-description, high action, high dialogue Literature, serialized over several volumes.
None of the above attributes makes it 'different' from serious Literature or heavy Literature though. I can't exactly tell you what makes a Light Novel a Light Novel but I can point out counterexamples to what is definitely not the definition of a Light Novel, and perhaps we can get a general sense of what it is from there.
So all these outliers merely make the proper definition of an LN hard for people not within the culture. The fact that many consider it a generic pop-medium and underrate the people within it indicates something. Its more like some kind of sub-cultural movement, like Alt-Lit, or New Weird, that encompasses everything that has been made in Japan so far. All of the above works, although completely divergent, have an awareness of things like otaku subcultural tropes, pop-tropes and genre-tropes, but the best writers will synthesize it into new forms. It can be a very post-modern, medium-defying genre. A new pop-art style of Literature? You can probably see reflections in people like Pynchon, for example, who took hardboiled and noir plotlines as well as plots from old adventure stories and the popular fiction of other eras and turned it into new matter for his books like Against the Day and Inherent Vice.
Who cares? No realy, book and story categorization is an effort doomed to failure (even though it does have uses). The main reason That categorization will fail is that stories are about people and people do not fit well in well defined boxes, and besides once you have a story that defines a category every other story in the category won't fit quite right even when an author is not trying to break categories. You are much better off with story description as adjectives are nonexclusive.
Let's give an example that I am very familiar with. These are the stories that used to be known as 'novel fantastic' before the categories split into science fiction and fantasy and then further split into space opera, cyberpunk, steampunk, high fantasy, epic fantasy, and so on and so forth ad-ridiculum. The splits are fuzzily identified by such rules as if it has spaceships or time travel it is sci-fi and if it has elves or dragons it is fantasy. Larry Niven Would beg to differ. He is a famous Science fiction author who made a serious effort to get the science right in every book he wrote but 'Rainbow Mars' Which Was written as a fantasy and included spaceships, NASA, a unicorn, and a time machine. So would Anne McCaffrey whose dragonriders of pern books which feature in addition to the aforementioned dragons features spaceships, computers, genetic engineering, orbital mechanics and time travel. We can also add Wen Spencer to the list Who in her Elfhome books has not only elves and spaceships, but she has magic and social networking, and she tops it off with dragons that teach quantum mechanics. Even Joss Whedon got into the act. His most famous category buster is Firefly which is a cross between a space opera and a western with cyberpunk and thriller influences.
There are two and only two questions that really matter: Is it a good book? Will it sell? Unfortunately the answers may differ.
Believe it or not, but this answer was inspired by this account.