"Tricky" refers to content that sexualizes minors or characters with the appearance of minors. Generally, these characters have a physical appearance that puts them noticeably under the legal age of consent in the United States.
With varying ages of consent across Japan, with the national minimum being 13, what passes as an acceptable age for sexual objectification of characters may be considerably younger than what many foreigners are accustomed to. Tricky refers to these instances of sexualized depictions of minors.
Jailbait generally refers to characters or persons who have not reached the U.S. age of consent but have a physical appearance that makes them indistinguishable from an adult. Tricky differs from jailbait in that the characters do not have the physical appearance of an adult and their age is most likely lower, generally from elementary school age to freshmen in high school.
The term was coined by Anime Vice staffer Tom Pinchuk during his review of Dance in the Vampire Bund, a series where the lead character is depicted as a pubescent female and often placed in erotic and/or sexual situations. Rather than use a term like icky, which could be considered offensive to the creators and fans of the series, Tom coined the phrase tricky to describe these situations.
"Lolicon" (loli for short) refers to sexual depictions of young female characters while "Shotacon" (shota for short) is used for male characters. The words are also used to describe people who enjoy these themes, typically as a fetish. Lots of anime and manga series feature token loli or shota characters, particularly in harem and/or ecchi themed series, but some series feature many of these characters and are even categorized as lolicon or shotacon series.