What Arunendra said is right for the button but for the hover you need some more code.
This can go in your .css file and can be styled any way you want, our you can put it this way in your .phtml depends on your needs
<style>
.tooltip{display: inline;position: relative;}
.tooltip:hover:after{background: #333;background: rgba(0,0,0,.8);border-radius: 5px;bottom: 10px;color: #fff;content: attr(title);
left: 20%;padding: 5px 15px;position: absolute;z-index: 98;width: 220px;}
.tooltip:hover:before{border: solid;border-color: #333 transparent;border-width: 6px 6px 0 6px;bottom: 6px;content: "";left: 50%;position: absolute;z-index: 99;}
.tip{display:inline-block;padding-left:12px;}
</style>
Then the button as mentioned in {theme}/template/catalog/product/view/addtocart.phtml
<?php
if($_product->getMybutton()){ //yes or no button
?>
<div class="tip">
<p class="tooltip">This can be anything you want and your text goes in ehre</p></div>
<?php } ?>