WooCommerce is the best WordPress plugin to build a webshop with.
WooCommerce is e-commerce plugin from WooThemes and is free to use.
"Where's the catch?" I hear you thinking, but there isn't one. WooThemes makes money from the paid plugins and templates.
The plugin has now been downloaded over 2 million times. More than 10% of all webshops/web shops use WooCommerce. This means the plugin is now used more than Magento, Zen Cart and OpenCart.

When WooCommerce and when not?

WooCommerce once started as a plugin for WordPress. Today, WooCommerce is part of the company behind WordPress. You will always run WooCommerce within an existing WordPress installation.
WooCommerce is mostly used from small to large webshops. It is free to use, many tips & tricks can be found online and products are easy to create and modify. Adding more extensive functions can be done with (mostly) paid plugins. You can purchase these plugins via Milcraft.