To sexually pleasure oneself to orgasm. Usually better than sex, because, not only do you know what you want better than a partner, there's no risk of STD or pregnancy! Everyone does it, though most people don't like to admit to it.
A popular hentai written by Mashumaro Jyuubaori. It depicts the sexual trials of a young girl named Alice in a magical land, similar to Wonderland, called Sexland. She is accompanied by her sexy female chronies the White Rabbit (Bunny) and the Chesire Cat (Cheshie). It is mostly yuri, and involves a lot of dickgirl sex.
Alice in Sexland is the best hentai I've read in years.