Project. This is a root node, that is created automatically when you start a new project. It is functionally identical to a Folder node.
Folder. It allows to group Folder and RequestStep nodes under it.
RequestStep. This is a node that you can use to send a request. It can have only one Assertion node as a child element.
Assertion. This one is used for writing tests. It can be a child node only for a RequestStep node.
Link. Allows to access an existing node.
Broken. It is used for descripting nodes that were loaded unsuccessfully. Can't be created manually and is not stored in the file system.