Pillar is a markup syntax and and a project with associated tools to write and generate documentation and books. It is written in Pharo and covered with many unit tests.
Basically it is the wiki syntax part from Pier extracted into an own project (so it can be used independent from Pier CMS).
But it also adds many new features like exporting into HTML, LaTeX and markdown. You can use the Mustache templating engine, configure numbering of sections, customize the export, syntax highlight code blocks, ...
If you already use the soon to be released Pharo 3.0 you can just load it from the config browser easily and try out.
While it is nice that you can use this from within your Smalltalk environment and access it from your projects code the idea is to not only use it from within the development image.
The package contains command line handlers to run it from a shell and Damien also prepares installable packages to make it run on platforms like Ubuntu. This should increase the end user experience for people who just want to use it for writing documentation or books without ever touching the implementation itself.
If you want to knnow more you should really read the excellent documentation and also check out the new cheatsheet summarizing the easy to learn syntax.