In the same way InDesign took on, and effectively beat, QuarkXpress, so FrameMaker was a direct rival to, and eventually outlived, Ventura. Adobe bought Frame Inc, the originators of FrameMaker, in 1995, and the first Adobe version was FrameMaker 5.
The key difference between FrameMaker and InDesign and the reason why Adobe can continue to run both applications, is FrameMaker's support for structured documents. It's ideal for creating books, particularly non-fiction works, and all kinds of technical documentation. It also supports XML, so is an ideal tool for creating documents which have a high and changeable data content, like timetables, price lists and catalogues.
I know the product well as, with two other authors, I wrote a commercial, 400 page book using nothing but FrameMaker 4. Even at that stage, its structured tools, using tags and frames, helped all three authors share chapter files and peer-review each other's work. Since then, Adobe has added many new features and broadened FrameMaker's application to online documentation, as well as paper-based documents.
FrameMaker looks more like Word than InDesign, and can be used as a sophisticated word processor. Extra DTP facilities, like the ability to anchor frames and pull-outs to words or phrases within text as well as to specific places on the page, make it very versatile.
The program also has particular strengths in paragraph and character styles, so you can establish complete, structured styles sheets, ideal for enforcing corporate style in in-house documentation. Its equation and formula support is better than almost any documentation program outside the specialist academic sphere and it handling of typography is elegant, without being too dogmatic.
There are some restrictions in comparison with InDesign, as tools are more strongly typed so, for example, you have separate tools for adjusting frame size and for editing frame contents. However, when you're working with more structured documents, this slightly more rigid approach can be a help.
Long document features, as you might expect, are a particular strength: things like automatic headers and footers, with content taken from chapter and paragraph numbers and titles, mean you don't have to keep them up to date manually. Tables of illustrations and figures, as well as tables of contents and indexing and straight tables themselves, are easy to set up and versatile in the layout formats you can impose on them.
FrameMaker 8 introduces a number of useful new facilities, among them Unicode, DITA support and the inclusion of rich media. Unicode, the character numbering system, which allocates a unique number to every character in every language, is a major aid in producing documents that will use multiple languages and character sets.