The axis of a node test determines what general category of nodes may be considered for the following node test. There are thirteen axes:
Ancestors of the current node
Ancestors, including the current node
Attributes of the current node (abbreviated "@")
Children of the current node (the default axis)
Descendants of the current node
Descendants, including the current node (abbreviated "//")
Elements which occur after the current node, in document order
Elements which occur before the current node, in document order (returned in reverse-document order)
The namespace nodes of the current node
The parent of the current node (abbreviated "..")
The current node (abbreviated ".")
The following graphic, courtesy of Crane Softwrights, demonstrates the axes graphically: