Shotacon (frequently shortened to shota) is a Japanese term referring to a fetish regarding young boys, or boys that attract such fetishization. Originally popularized in BL fandom as the bishonen aesthetic taken to its natural extreme, but more generally refers to an attraction to a male character around 13 years old or less. Comes in hetero-sex and same-sex flavors, and is found in both female-oriented (especially Yaoi) and male-oriented works.
The earliest Boys Love manga of the early 1970s featured love affairs between early-teen boys, but were marketed to (female) readers of the same age. Shotacon first became an identifiable genre in the Boys Love Doujinshi community of the early 1980's, but the idea was quickly picked up by male fans (who took it in a substantially different direction). By the early 1990s, when the name was standardized, the male shotacon Doujinshi community was about the same size as the Boys Love shotacon Doujinshi community.
Currently, Japanese fans and publishers use term shonen ai (boy's love) to designate specialty shotacon works (for either gender, although it is more commonly used for male-oriented works), which is at variance with its general Western meaning of nonexplicit Boys Love. Works marketed specifically as shotacon vary substantially by gender target: Boys Love shonen ai shotacon is generally romantic, may or may not be explicit, and follows Boys Love tropes; shonen ai shotacon for men is usually sexually explicit, tends to overlap heavily with Futanari and Lolicon, and typically features lots of crossdressing.
Shotacon is also used as a theme in works for a more general audience, where it tends to be less controversial than Lolicon. In works aimed at a female audience, it's typically played for Moe Fan Service, or to characterize someone as a Covert Pervert or Yaoi Fangirl. In works for a male audience it's typically played for humor; because boys are stereotypically more obsessed with sex, shotacon themes are generally intended to sexualize the woman, rather than the boy who is the target of her affection. So it pops up in Shonen a lot, even if those boys are on the technical borderline; some readers find the idea of physical encounters with older women enticing, while others like the aggressive female characters usually involved. If a show has an aggressive, sex-hungry Ojou with an annoying laugh, she's the most likely candidate, although usually only if potential male targets are very limited.
The name is a portmanteau for Shotaro complex (by analogy to Lolicon, "lolita complex"), which refers to Shotaro Kaneda, the main character of Tetsujin 28, a Cute Responsible Kid whom many people found appealing.