Javascript Page Document ready, PageLoad, Application Init

$(document).ready()

• Ideal for onetime initialization.
• Optimization black magic; may run slightly earlier than pageLoad().
• Does not re-attach functionality to elements affected by partial postbacks.

pageLoad()

• Unsuitable for onetime initialization if used with UpdatePanels.
• Slightly less optimized in some browsers, but consistent.
• Perfect for re-attaching functionality to elements within UpdatePanels.

Application.Init

• Useful for onetime initialization if only ASP.NET AJAX is available.
• More work required to wire the event up.
• Exposes you to the “sys is undefined” error if you aren’t careful.

Resource: http://encosia.com/document-ready-and-pageload-are-not-the-same/