When the browser triggers an event or other JavaScript calls jQuery's .trigger() method, jQuery passes the handler an Event object it can use to analyze and change the status of the event. This object is a normalized subset of data provided by the browser; the browser's unmodified native event object is available in event.originalEvent. For example, event.type contains the event name (e.g., "resize") and event.target indicates the deepest (innermost) element where the event occurred.
By default, most events bubble up from the original event target to the document element. At each element along the way, jQuery calls any matching event handlers that have been attached. A handler can prevent the event from bubbling further up the document tree (and thus prevent handlers on those elements from running) by calling event.stopPropagation(). Any other handlers attached on the current element will run however. To prevent that, call event.stopImmediatePropagation(). (Event handlers bound to an element are called in the same order that they were bound.)
Similarly, a handler can call event.preventDefault() to cancel any default action that the browser may have for this event; for example, the default action on a click event is to follow the link. Not all browser events have default actions, and not all default actions can be canceled. See the W3C Events Specification for details.
Returning false from an event handler will automatically call event.stopPropagation() and event.preventDefault(). A false value can also be passed for the handler as a shorthand for function(){ return false; }. So, $( "a.disabled" ).on( "click", false ); attaches an event handler to all links with class "disabled" that prevents them from being followed when they are clicked and also stops the event from bubbling.
When jQuery calls a handler, the this keyword is a reference to the element where the event is being delivered; for directly bound events this is the element where the event was attached and for delegated events this is an element matching selector. (Note that this may not be equal to event.target if the event has bubbled from a descendant element.) To create a jQuery object from the element so that it can be used with jQuery methods, use $( this ).