What difference between $ and $() in jQuery

What difference between $ and $() in jQuery

According to jQuery API I see that $() is collection of matched elements.
But what is $ ? Example from imagesLoaded library below.
if ( $ ) {
$.fn.imagesLoaded = function( options, callback ) {
var instance = new ImagesLoaded( this, options, callback );
return instance.jqDeferred.promise( $(this) );
};
}