As you may have heard from Scott Guthrie or Scott Hanselman, Microsoft announced that it will make jQuery part of the official dev platform. JQuery will come with Visual Studio in the long term, and in the short term it'll ship with ASP.NET MVC. Microsoft will also ship a version of Visual Studio which includes Intellisense.
A big part of the appeal of jQuery is that it allows you to elegantly (and efficiently) find and manipulate HTML elements with minimum lines of code. jQuery supports this via a nice "selector" API that allows developers to query for HTML elements, and then apply "commands" to them. One of the characteristics of jQuery commands is that they can be "chained" together - so that the result of one command can feed into another. jQuery also includes a built-in set of animation APIs that can be used as commands. The combination allows you to do some really cool things with only a few keystrokes.