As a frequent user of this application, I have grown to really appreciate it. Therefore I feel the need to elaborate on Saige's description.
FrameMaker is a desktop publishing application published by Adobe. It's widely used be tech writers because it's very well suited for their sorts of tasks. If you need to manage the content of a large user's manual and most of your graphical content consists of line art diagrams of things, then FrameMaker is an excellent choice. However, if your desktop publishing needs are more along the lines of doing layout for more graphically intensive applications (such as producing brochures, magazines, etc.) then you should probably look into using PageMaker instead.
FrameMaker is very good at letting you manage the content of your document, and that's its real power. For names that might change during the life of a document (e.g. product names, interface names, etc.), you can define variables so that later on you only have to change one instance of the name in order to propagate the change throughout the document automatically. If paragraphs in one part of a document reference another part of the document by its section number, you can define variables for such references so that they are automatically updated as you insert or remove new sections (thus altering the section numbering). You can also do things like automatically generate a table of contents, or split your document up into multiple files and have them grouped together into a single project; all cross referenced if you so desire.
And this program seems to scale really well. If your documents are hundreds of pages long, you can still manage them nearly as efficiently as if they were only a couple pages long. Compare this to less professional word processing programs like Microsoft Word which circa version 6.0 still had O(n^2) sort and search algorithms within it that made any document beyond about 20 pages pretty unmanageable.
If you happen to have access to it, and you want to try running it, beware that the interface you get might seem very unique and less than intuitive if you're used to a lot of modern word processing programs. For example, to save your document you first press the 'esc' key, then the 'f' key, and then the 's' key. That might seem strange because most people are used to something like simultaneously holding down the 'alt' and 's' keys to save, or something along those lines. When you look around the menus, you'll note cryptic strings next to each menu item like !fs, !cp, !cb, and so on. The '!' refers to the 'esc' key, and the shortcut for that menu item is to type in that cryptic string (with the appropriate substitution of the 'esc' key for the ! symbol, of course).
But if you want to produce really slick documents with lots of color graphics, you'll probably want something else. With enough hard work and patience, you can still accomplish such things with FrameMaker. It's just that you'll likely waste a lot of time trying to conform the functionality of FrameMaker to suit a task it wasn't really designed for. And you probably won't be making good use of the special features that a large technical manual would be utilizing. For example, does your half-page, glossy flyer for an upcoming rave need a table of contents?