What is the DITA Open Toolkit, anyway?
The DITA Open Toolkit is a popular, free, open-source tool used to transform DITA documents and maps into the output document formats you desire. In fact, most of the proprietary authoring tools use the DITA-OT to transform DITA documentation projects, so you aren't wasting your time learning about how it works. Many errors are more quickly fixed if you understand what the toolkit is doing "beneath the hood" of your authoring tool.
DITA-OT uses Ant to generate your documents. The primary ant script is build.xml, which imports several other build scripts to initialize, validate, and transform your .dita documents.
See Introducing Document Generation for more information about how DITA-OT processes documents.