If you are a web developer then you MUST use jQuery for writing java script. There is no excuses for not to use jQuery and I personally feel it’s a big mistake if jQuery is not used.
You want to ask me “Why should I use jQuery?”. Well jQuery is lightweight, awesome, brilliant, exceptionally good framework for accomplishing complex java script tasks in a single line of script.(follow this link to learn more about jQuery)
Memorizing the hundreds of jQuery selectors, methods and event handlers can melt your brain. So here is the cheat sheet that can sit on your desktop as a wallpaper and be handy for your reference(click on the image to enlarge)