Or International Youth Congress. An annual convention targeted at Esperantists under 30 and organized by TEJO (The World Organization of Young Esperantists). The first, then called the Internacia Junulara Kunveno (International Youth Convention), was in 1938 in Groet, Netherlands. For reasons which should be obvious, it did not occur again until 1946, when it got its current name.
Generally IJK takes place in the week preceding the Universala Kongreso, in the same country but a different city. Like UK, it has a theme every year; by way of example, this year's is "Language, Culture, and Opportunity". Also like UK, it is run primarily by the local Esperanto group(s), who arrange the program for the week. As such the content varies; however, it generally includes music, theater, dancing, and outings to assorted pretty places.
Notable IJKs include the 36th IJK in Rauma, Finland, which produced the Rauma Manifesto, criticizing the goals of the Esperanto movement at the time and arguing for the acknowledgement and development of Esperanto culture ( and yes, there is such a thing :) ).