IFID stands for Interactive Fiction IDentifier. An IFID is a serial number that's assigned to each work of Interactive Fiction. Each work has its own unique IFID, giving players, authors, and archivists a universal, unambiguous way to refer to a given game. It's the same idea as the ISBN system for books. The IFID system is defined by the Treaty of Babel, which was created in 2006.
If you're the author, you're responsible for creating an IFID for your game. Most of the current authoring systems have built-in tools that do this for you when you create a new project. If you're not sure how this works in your system, the Treaty of Babel has the technical details.
If you're not the author, there are software tools that can find the IFID for a given story file. The Treaty of Babel site has a free, portable program that does this. Some author systems also include built-in IFID extraction tools (for example, in TADS Workbench on Windows, the Tools > Read IFID From... command can extract the IFID from any story file, even for non-TADS games).
Yes. The IFID for an older game (written before the IFID system was created) is based on the binary contents of the game's published story file. If a pre-IFID game has been released more than once - for version updates or ports, for instance - each release will have a separate IFID, because each release's binary file contents will be at least slightly different.
A newer game with an explicitly assigned IFID should only have one IFID for its entire life cycle, no matter how many times it's published. This is because an explicitly assigned IFID is stored directly in the story file, so the IFID extraction tool will find the assigned IFID and won't have to calculate an implied IFID based on the file contents. So, even when the game has been published in multiple forms, it will always have the same IFID.
All of them. IFIDs are intended to permanent, universal identifiers, so once a game is released with an IFID, that IFID forever refers to that game in all of its versions.
This means that you should generally never delete or change an IFID in a game's listing. The only exception is when an IFID in the listing is wrong - if it has a typo, or someone accidentally entered an IFID for a different game, then the erroneous entry should be corrected or deleted.
But once a game is released with a given IFID, that IFID should stay in the game's listing, even if a new release has a different IFID. In this case, you should simply add the new release's IFID to the listing, and keep the old one(s). The reason is that the old version won't just disappear from the planet just because a new version came out - the old version will still be out there in on-line archives, individual hard disks, etc. Someone who has a copy of the old version might someday want to find more information about the game. To do this, they can extract the IFID from their copy using one of the Babel tools, then they can come here and look up the IFID in our database. They'll find what they're looking for, so long as the game's listing includes all of the game's past and present IFIDs.