$("html_element")
for HTML-Tag`s
$("*")
for all elemet`s in the HTML - document
$("#element_id")
for HTML-Tag with an ID-attribute
$("")
referes to jquery-Object
document.getElementById()
referes to a HTML-Dom-Object
With the HTML-DOM-Object you can only use "normal" Javascript. With the jQuery-Object you can use normal javascript but you can also use the hole jQuery-Framework with all methods implemented in the Framework
$ is not defined
If this Error- Message appears, check if the jquery .js File is implemented in your html head-Tag