How to get jQuery to wait until an effect is finished?

You can specify a callback function:

$(selector).fadeOut('slow', function() {
    // will be called when the element finishes fading out
    // if selector matches multiple elements it will be called once for each
});

Documentation here.

With jQuery 1.6 version you can use the .promise() method.

$(selector).fadeOut('slow');
$(selector).promise().done(function(){
    // will be called when all the animations on the queue finish
});

You can as well use $.when() to wait until the promise finished:

var myEvent = function() {
    $( selector ).fadeOut( 'fast' );
};
$.when( myEvent() ).done( function() {
    console.log( 'Task finished.' );
} );

In case you're doing a request that could as well fail, then you can even go one step further:

$.when( myEvent() )
    .done( function( d ) {
        console.log( d, 'Task done.' );
    } )
    .fail( function( err ) {
        console.log( err, 'Task failed.' );
    } )
    // Runs always
    .then( function( data, textStatus, jqXHR ) {
        console.log( jqXHR.status, textStatus, 'Status 200/"OK"?' );
    } );

if its something you wish to switch, fading one out and fading another in the same place, you can place a {position:absolute} attribute on the divs, so both the animations play on top of one another, and you don't have to wait for one animation to be over before starting up the next.