I have no idea what Pollen does. The homepage really doesn't explain anything, and even after skimming the tutorials and the quick tour I still don't know.
Is it a static site generator? A blogging engine? Does it create PDFs? Epub? I have no idea.
I think of it as LaTeX for HTML. You write markup and Racket code that produce websites; the author used it for his web book Practical Typography: http://practicaltypography.com/
Since you can write arbitrary Racket, you can create markup for anything you want. Write some code that automatically generates plots, formats code examples, makes custom asides and footnotes -- just like you can write arbitrary macros for LaTeX to do all sorts of things. Except with Racket instead of TeX macro goo.
It natively outputs HTML but you could have it render anything vaguely XML-y.
At the moment it feels more like TeX than LaTeX, since it provides the low-level features but not a default setup for building specific things. I'd like to try it to write a website, but there's a steep startup cost of reimplementing all the basic macros first.
The author should copy this in the site. The information is available in the menu, but you must enter in each one and read them to understand the general idea.
Which could be, as the GP said, a static site generator, blogging engine, ePubs, etc.
If your one-line intro for anything includes the word "system", stop and rewrite. For example:
Pollen helps authors create beautiful and functional web-based books that are written as programs that generate HTML.
Never-have-I-ever seen anything that confuses "feature" and "benefit" so much as this project. Why do I want my next book to be a program? A 20-minute video explaining it is almost condescending.
And when someone does that, people here complain about reading through lengthy introductions before they see any code. I presume it's a misguided attempt to satisfy such people that the book goes Installation, Quick Tour, then Backstory (before the Tutorials). In the end, it's not worthwhile to cater to this sort of "reader".
If you "skimmed" the tour and tutorials and don't know what it is, stop skimming. Then, start reading.
This isn't some arcane library you need domain-specific knowledge to understand, it's a system with a purpose and intent that's quite explicitly explained.
I don't think I can forgive the use of ◊ as an escape character, though. It is not on any commonly used keyboard, so unless the intent was to make users avoid it as much as possible, it's just too awful to put up with.
Please note that on OSX you have to manually add the Racket binaries to your path. If you use zsh and Racket v6.2 for instance you could add the following to your .zshrc: export PATH=$PATH:/Applications/Racket\ v6.2/bin
It needs a front end. If this is the web version of TeX, it needs LaTeX and TeXshop or Kile. You're not suggesting that text be originally written in Pollen, are you?
Pandoc is about reading lots of formats (eg Markdown) and writing lots of formats (eg epub, pdf, html). It has enough config options that you can reasonably produce books with it.
Pollen is like TeX. It has one input format, which is a Turing-complete programming language. It can output a limited number of formats. You have much more control about the output than with Pandoc, so can do visually pleasing typography.
Is it a static site generator? A blogging engine? Does it create PDFs? Epub? I have no idea.