how to get the class object dynamically in jquery code example

Example: how to get the class name dynamically using jquery

<input id="btn" class="wpsc-buy-now-button wpsc-buy-now-button-64" type="image"
    name="submit" border="0" src="https://www.paypal.com/en_US/i/btn/btn_buynow_LG.gif"
    alt="PayPal - The safer, easier way to pay online" />