These distributions are all available for download from http://sourceforge.net/projects/dita-ot.
The only other software required to do DITA processing with this package is the Java J2SE SDK. You may optionally want to install the Microsoft HTML Help compiler and the JavaHelp processor.
This distribution contains only DITA Toolkit. All the required and optional processors and tools must be installed separately to create a functioning build environment. Prior releases of the Toolkit were distributed only in this way. You might want to download this distribution if you have a prior version of the Toolkit already installed, since the release 1.3 Toolkit prerequisites are still the same as those for release 1.2.2. The small distribution is also typically the one used to embed the Toolkit in other products.
The source distribution contains the source and executable code for the Toolkit (it also contains the source code for this document). You might download this distribution if you need to modify Toolkit Java code or if you want a detailed look at how the Toolkit works.