They generally serve as meeting places for system and network administrators, hackers, and computer security experts.
Typically the actual details of any given convention are couched in mild secrecy due to the legality of certain panels, as well as the willingness of attendees to explain themselves to law enforcement and less computer-savvy individuals (see hacker definition controversy).
Common topics include wardriving, lockpicking, corporate and network security, personal rights and freedoms, new technologies, as well as general 'geek' motifs.
While many conferences use a fairly standard paper/plastic badge to identify attendees by name, many hacker con's have evolved to use more non-traditional badges, such as electronic PCB's with LED's, LCD screens, and can include wifi and advanced, often hidden functionality, to include games, that do not identify the individual, sometimes promoting a group identity or regional/local group affiliation.
Usually this is done in a client-server architecture with multiple clients who send individual pixels and the server displays.