An incarnation of pure evil in the guise of a software installation program. Created by the InstallShield Software Corporation (http://www.installshield.com), which describes itself as "a leading provider of professional Windows software development tools." Best known for programs such as InstallShield Professional and InstallShield for Windows Installer ... of Doom.
Contrary to chaosdiscord's writeup, InstallShield is not the actual programming language. InstallScript is the scripting langauge in which InstallShield installations are written. It is similar to C and related languages, but much more diabolical. InstallScript consists mostly of functions, which are used to create event-driven scripts that run setup, configuration, and uninstallation procedures. (And you can control uninstallation events, but it often does leave files or registry entries behind after uninstallation.) Frequently, these functions are difficult to understand at best, and require advanced degrees in programming, engineering, physics, higher math, and/or a background in interpretive dance.
Anyone who has ever been unfortunate enough to have been required by an employer or other entity to create or modify an InstallShield project will understand what I am talking about, even if they do not wholly agree with my assessment. While, in essence, InstallShield is a nifty little tool for creating installations, it often breaks, does not do what you expect/want it to, and may even cause damage to existing software or operating systems, for no apparent reason. And of course, like other programming languages, leave out one little semi-colon and watch the compiler blow up, but God forbid it should clue you in to what you did wrong, or where your syntax error is (frequently prompting me to scream obscenities at my computer and bitch about how the stupid code won't compile). Furthermore, trying to talk to their support staff is like trying to squeeze orange juice out of a lemon. You can almost get them to understand you, if you're lucky enough to get someone who speaks fluent English, and you can come close to a resolution, but you can never quite achieve your goal of actually solving your problem. However, they may insult your intelligence several times in the process, if you like that sort of thing.
Luckily, there are newsgroups and a website to bitch about the inefficacy of InstallShield, and to beg other users for help. You can try the news.installshield.com server, which offers such fascinating lists as installshield.is6.general, installshield.is6.installscript, and even the installshield.is6.wishlist, where you can fervently pray and sacrifice small woodland creatures (or your sanity/job security/hard drive, whichever is more highly valued) to the demi-gods of InstallShield to enhance crappy features, and fix age-old bugs (which are, not surprisingly, just called "features"). There's also http://www.installsite.org which maintains lists of bugs and fixes, software updates, and a searchable support database. It is not run by InstallShield; therefore, it may actually provide some insight and help.