Javascript auto popup (without clicking a button)

I was just wondering if it is possible to display a javascript message (either popup or something nice) which is triggered by something which isn't a mouse click, so for example:

<?php if(a == b) {
    Then auto pop up something
}

I have had a look around and cant seem to find anything, also how do people make like stylistic popups like.. i guess maybe CSS but really fancy ones which cant be done with CSS any ideas?

Thanks a bunch :)