Of course, there are lots of tutorials around for this, this is mainly for reference

$.fn.myFunction = function()
{
    // use the following loop to iterate through all objects
    // returned by the jQuery selector which was used
    return this.each(function()
    {
        // here we can access any properties using
        // the normal jQuery selectors
        alert($(this).html());
        $(this).addClass('red');
    });
};

So this function alerts the HTML of the selected element(s) and adds a new class ‘red’. You can then call the new function using:

$('#myElement').myFunction();

Another example of how easy and powerful jQuery is Look here for more information.