In GIMP terminology, a filter is a plug-in that modifies the appearance of an image, in most cases just the active layer of the image. Not all of the entries in this menu meet that definition, however: the word "filter" is often abused to mean any plug-in regardless of what it does. Indeed, some of the things you will find in this menu do not modify images at all.
With the exception of the top three items, all of the entries in this menu are provided by plug-ins. Each plug-in decides for itself where it would like its menu entry to be placed. Therefore, the appearance of this menu can be completely different for each user. In practice, though, the appearance does not vary all that much, because most plug-ins come with GIMP when it is installed, and of course they are always in the same places in the menu.
Plug-ins are not restricted only to the Filters menu: a plug-in can request to be placed in any menu. Indeed, a number of GIMP's basic functions (for example, "Semi-flatten" in the Layer menu) are implemented by plug-ins. But the Filters menu is the default place for a plug-in to locate itself.
For general information on plug-ins and how to use them, see the section on Plugins. Information on the filters that are provided with GIMP can be found in the Filters chapter. For filters you install yourself, you will of course have to obtain information elsewhere.