Well if you open their offical websites you can see the difference. With Joomla or Drupal you can create any kind of website . Magento is specialized for Oscommerce im not telling Joomla cant do it becouse also Joomla have some components for oscommerce like virtuemart. If you want to create a oscommerce website better use Magento or another CMS who is specialized for Oscommerce.
Although Magento is specific to eCommerce, Drupal has many modules for setting up an eCommerce site with, including http://drupal.org/project/magento so you get the power of Drupal (and it's a very powerful CMS/CMS framework) and an eCommerce site all in one. You need to evaluate what your needs are evaluate them against each CMS.
[FONT=Arial, sans-serif]Joomla is not an ecommerce but it can be with installing a shopping cart extension. IF you are going for an ecommerce website better use magento. If you are going for your business website and selling a couple products on it its ok to use joomla.And drupal is not a CMS it's a framework. You can build a CMS using Drupal. If you need simple CMS don't use Drupal. If you are building bigger site then Drupal gives you a lot of advantages.[/FONT]
I have already used Drupal, and must admit it was a pleasant experience. The Commerce Guys have create a commerce component for Drupal - Drupal Commerce. So, this CMS can also be used for online stores. Magento is for advanced users. If you are going to build an ecommerce website, you'd better try something like Prestashop or Opencart. As for Joomla, I didn't work with it yet. But I know it also has e-commerce solution - Virtuemart. Nowadays, VirtueMart is the leading Online-Shop Solution for Joomla. Regards, Leis
Virtuemart can be slightly frustrating especially when trying to find which files to edit but overall it's a very powerful shopping cart ... and its free!
If you want to compare magento to drupal with ecommerce in it, try commerce kickstart distribution. It is Drupal and fine tuned to be a ecommerce site with commerce module acting as it base.