Magento: Back Button On Product Detail Page

"/app/design/frontend/default/default/template/catalog/product/view.phtml"
path for view.phtml can be diffrent for diffrent magento themes.

and put this code

<?php if ($_category = $_product->getCategory()): ?>

<div class="back-button">
    <button onclick="history.back();" class="button" title="<?php echo $this->__('Back') ?>" type="button">
        <span>
            <span><?php echo $this->__('Back') ?></span>
        </span>
    </button>
</div>
<?php endif; ?>