Notification templates allow you to control exactly what is sent for each notification type. There are 3 template formats:
Plain: used for private messages and plain text emails
HTML: used for HTML emails
Simple: used for XMPP
Editing a template allows you to change each of the formats using the DotLiquid markup syntax.
Below the input box is a list of variables that are available to be used in the template. These are specific to the notification type, so, for example, build-related notification types will give you access to the build's status, changesets etc.