Short answers, pulled from the story.
A light novel is a type of popular Japanese fiction, typically targeting readers from their teens through twenties or older, published in the small bunkobon paperback format measuring 10.5 by 14.8 centimetres. It is distinguished by manga-style illustrations, usually in black and white, and a low price point. The format is frequently adapted into manga, anime, and live-action films.
The term was coined around 1990 by Keita Kamikita, the system operator of a science fiction and fantasy forum. He chose the label deliberately to avoid calling the illustrated novels young adult fiction, since they attracted readers across multiple demographics.
By 2009, light novels generated ¥30.1 billion in annual sales, representing about 20% of all bunkobon format paperback sales in Japan. Kadokawa Corporation's publishing subsidiary holds a 70-80% share of the market through imprints such as Dengeki Bunko and Kadokawa Sneaker Bunko.
Sword Art Online began as a web novel published online in 2002 before a publisher brought it to print. Its commercial success popularized isekai as a recognized genre and contributed to the growth of write-your-own fiction websites in Japan, as well as increased light novel popularity in Western markets.
The two largest English-language publishers are Yen Press, a joint venture between Kadokawa and Hachette Book Group, and Seven Seas Entertainment, which in April 2007 became the first English publisher to print light novels in their original Japanese bunkobon format. Other publishers include Viz Media, Vertical, J-Novel Club, and One Peace Books.
Isekai, meaning "different world" stories, follows an ordinary person transported from modern city life into a fantasy and adventure setting. Sword Art Online's success helped spread isekai as a distinct genre, and subsequent works including KonoSuba, Overlord, and Re:Zero grew more popular in part because of it.