you mean a button in your application? "target" is also a param of the getURL function, so what do you mean exactly?
if you want to know about the param "target":
it's the same as in html: "_blank", "_parent", "_self"
if you want to know how to create a button:
1. make a textbox and type some text in it.
2. convert this textbox to a button.
3. click on your scene, and then back on the button. Name the button in the properties window.
4. Open the "action"-window of the frame your button is located in, and type the following code:
Hope this makes it clearer. if not, just ask
UnrealEd