Why do I like jQuery over say prototype/Scriptaculous? At a very basic level, it probably comes down to personal preference, jQuery was easier for me to grok. But what first got me hooked was its straightforward use of selectors to work on DOM elements.
jQuery selectors are a combination of CSS 1-3, XPath, plus some custom code to glue it together. Essentially, the best parts from both of these query languages were taken, combined, and used to create the final jQuery expression language. If you already know CSS (which most web developers do) then you’re going to be fine.