Every TemplateBasedVM in Qubes is, as the name implies, based on some TemplateVM.
The TemplateVM is where all the software available to TemplateBasedVMs is installed.
The default template is based on Fedora, but there are additional templates based on other Linux distributions.
There are also templates available with or without certain software preinstalled.
The concept of TemplateVMs is initially described [here](/getting-started/#appvms-qubes-and-templatevms).
The technical details of this implementation are described in the developer documentation [here](/doc/template-implementation/).
Some templates are available in ready-to-use binary form, but some of them are available only as source code, which can be built using the [Qubes Builder](/doc/qubes-builder/). In particular, some template "flavors" are available in source code form only. Take a look at the [Qubes Builder documentation](/doc/qubes-builder/) for instructions on how to compile them.
These templates are supported by the Qubes community. Some of them are available in ready-to-use binary package form (built by ITL), while others are available only in source code form. In all cases, ITL does not provide updates for these templates. However, such updates may be provided by the template maintainer.
By installing these templates, you are trusting not only ITL and the distribution maintainers, but also the template maintainer. In addition, these templates may be somewhat less stable, since ITL does not test them.