A form ID is an object property assigned by creators or the engine to each and every thing in Fallout 3, be that items, NPCs or parts of the world like sign on a wall or the wall itself. There are two types of Form IDs: Base IDs and Reference IDs (usually shortened to Ref IDs).
Base ID is the number assigned to a template for an object that is used to create many instances of that object. For example all bottle caps in the game have exactly the same Base ID. This ID is used in scripts or console with commands that create new instances of object, like
In Fallout 3 "GetBaseObject" or "gbo" can be used to find the BaseID of an object in-game. This is a FOSE command, according to http://fallout.wikia.com/wiki/Talk:Form_ID , and so FOSE (Fallout Script Extender), found at http://fose.silverlock.org/ , must be installed to use it.
The Reference ID is the unique ID of an individual object (unlike the Base ID, which is an ID for an object template). For example, all the bottle caps created from same Base ID will have different reference IDs. This ID is used to manipulate existing objects with commands like
move to, or
prid, for example.
Any item that is not created by pre-defined game scripts (either original or from mods) will have reference ID starting with FF to indicate that this item belongs to this particular save game. It is for dynamically generated items, like loot from containers, random encounters or trader's wares.
Load order of modules (ESMs and ESPs) will affect the ID number of modules. The first two digits of a ID number corresponds to its load order (in Hexadecimal, like the rest of the number). One must use a utility like FO3Edit to ascertain the load order of a module. Fallout3.esm's ID number is should always be 00, as it should always be the very first module to load. The ID number series in the FF (decimal equivalent: 255) range is reserved by the game engine for objects created and saved in the gamesave file (such as PlaceAtMe'd objects, projectiles, dropped inventory, or list-spawned actors).
According to the layout of this system, the maximum number of modules that can be loaded by the game is 254 (256 load order ranges, - 1 for the Savegame FF range, - 1 for the always-mandatory Fallout3.esm).