eCommerce refers to online stores where customers can purchase items on your website. There are many services that make it 'relatively easy' to get a shop up and running such as Volusion, Shopify, and Bigcommerce. There are also many ways to install e-commerce onto an existing CMS such as Woocommerce for Wordpress, Virtuemart for Joomla, and Ubercart for Drupal. This could potentially allow a person to keep the power and functionality of their CMS of choice and add a shopping platform on top it.
Now, here's where I explain the 'relatively easy' qualification I made above. All of the cart options I listed will get you up and running with the generally accepted features needed. However, as soon as you need something special (and everyone does), it can get really nasty really fast. I've had clients expect me to rebuild them Amazon by using a free component such as Virtuemart, and it's just not going to happen. There are a ridiculous number of hidden difficulties in e-commerce such as complex shipping rules, differing tax rates, inventory tracking, user management, security protocols and procedures for handling credit card information, obscure payment gateways, coupons, trial periods, and on and on. These complexities often get overlooked by clients or eager salesmen and the result is rarely pleasant. Trying to 'force' Woocommerce or Volusion to do something it's not built to do opens a big nasty can of worms.
My favorite tool so far when it comes to building an eCommerce shop is Shopify. One of my favorite aspects of Shopify is that they give you direct access to the code that populates the various pages of the shop. They also use their own templating language know as 'liquid' that allows you to iterate through each of the products, categories, pages, blog entries, etc. In addition, the platform is highly extendable so when custom needs arrive, additional functions can be build on the platform.
Of course this is just one way of handling e-commerce, and there are many other routes out there I can help you with. If you're interested in getting your very own online shop, drop me a line.