From the context of the code I am reading, it seems like $("<tag></tag>") creates a tag, where as $('<tag>') is a selector that searches for a tag. What's going on here? Actually I might not have the syntax of the second one right, but I'm sure I've done $('idName') like this before.
What's going on?
Create an element:
$("<div>Hello</div>").appendTo("body"); // <body><div>Hello</div></body>
Select the DIVs:
$("div").addClass("myClass"); // <div></div> -> <div class='myClass'></div>
Select an element by its ID:
$("#myDiv"); // selects <div id='myDiv'>Hello World</div>
And by its class:
$(".myDiv"); // selects <div class='myDiv'>Hello World</div>
There's no difference between double quote and single quote.
Jonathan gave you a good example of the different between selector and and DOM element creation. It can be done with this too:
Create an element:
$('<div></div>');
$("<div class='class1'></div>");
$('<div attrib="val"></div>');
Select the DIVs
$('div');
However $('<tag>') DOES NOT select a tag. Refer to the docs.
The difference between $("<tag></tag>") and $("<tag>") and $("<tag />") is personal style/preference (unless you're using IE, apparently. see comment). All three will use the native createElement() method to create a now DOM element. If the tags have parameters it will parse them create the element by some other means.
More info on creating things here: http://api.jquery.com/jQuery/#jQuery2
To select elements by id, you'd use $("#idName").
To select existing elements by tag name, $("tag").
To select existing elements by class name, $(".className").
More info on selecting things here: http://api.jquery.com/jQuery/#jQuery1