<p>I've seen some posts where jQuery has been favored vs ExtJS. I haven't looked at jQuery in detail, but from what I read so far, jQuery doesn't provide the kind of UI which comes with ExtJS. Am I correct? Why would some of you prefer jQuery in ASP.NET?</p>

<p>Thanks</p>

There are two schools of javascript frameworks, ones that focus on widgets (Yui, ext, etc) , and ones that focus on behavior (jquery, prototype, moo, etc)
JQuery just makes life easier to build dynamic, sexy sites. If you are just doing system.draggy.droppy asp development, you can ignore both, since you probably aren't really touching javascript at all. But if you do use javascript, it is worth your time to learn one of the frameworks that are out there, and jquery is currently the most popular.