Three top features of jQuery: the multitude of selectors, manipulating attributes, and tacking Ajax/JSON requests. Choses jQuery over Prototype
As well as supporting CSS based selectors, jQuery also supports XPath expressions. This page lists a plethora of code snippets demonstrating jQuery's XPath matching capabilities.
A list of the CSS Selectors supported by jQuery. It includes the regular gamut of CSS selectors as well as attribute selectors, pseudo-selectors and child selectors.
A useful overview / introduction of the jQuery library, covering using selectors and XPath expressions to target specific elements in a document, simple Ajax requests, the basic animations available, restructuring a document, creating jQuery plugins.