If you're going to start a thread about every single XPath features, I suggest you first go through an XPath tutorial with examples, e.g. :
Get back here with further questions if you don't understand a specific point.
Identifies all nodes that appear after the current node, excluding its descendants, in document order.
Identifies all sibling nodes that follow the current node.
Identifies all nodes that appear before the current node excluding its ancestors, in reverse document order.
Identifies all sibling nodes that precede the current node.