Form ID

is a object property assigned by creators or engine to each and every thing in Fallout 3, be that items, NPCs or parts of world like sign on a wall or the wall itself.

Base ID
Base ID is the number assigned to template of each 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  or.

Reference ID
Every new created item have its own reference ID. For example, all the bottle caps created from same Base ID will have different reference ID. This ID is used to manipulate existing objects with commands like,  , or  , 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.