XPath is a language that enables the pinpointed querying of elements in an XML document, serving as a flexible tool for traversing the DOM of these documents, similar to HTML documents. It allows developers to locate elements using both absolute and relative pathing. Absolute XPath expressions begin with the root element or a single forward slash, indicating a direct path to the element, while relative XPath expressions start with two forward slashes, allowing navigation from anywhere in the document. Using relative XPath can prevent changes elsewhere in the document from breaking the expression. The components used to construct an XPath expression include the `/` or `//` indicator for navigation, the element type (tag name), predicate (attribute value), and axes (parent, child). Additional functionality available through XPath includes functions, operators, and axes that simplify operations such as locating elements based on attribute values or selecting parent or child elements. When used with Selenium, XPath enables efficient element location in web application test scripts, allowing for thorough testing of critical web application functionality.