What is Magento you ask? This is an excellent question. The quickest answer is that Magento is a platform for building eCommerce sites. If you want to sell something online Magento is among the fastest ways you can make that happen.
There's certainly more to it than that though. If you go to the Magento site this is what they'll tell you...
"A. Magento is a feature-rich eCommerce platform built on open-source technology that provides online merchants with unprecedented flexibility and control over the look, content and functionality of their eCommerce store. Magento’s intuitive administration interface features powerful marketing, search engine optimization and catalog-management tools to give merchants the power to create sites that are tailored to their unique business needs. Designed to be completely scalable and backed by Varien's support network, Magento offers companies the ultimate eCommerce solution."
Still not clear? Try thinking of Magento as an iPhone.
When you buy an iPhone it does the basics; makes calls, stores contacts, gets you online. You're certainly not going to go out and make your own phone so it makes sense for the vast majority of people to go buy one that already exists. The same is true for a website. You want something that already has checkout and cart functionality, can store product information, remember customers, etc.
The other great thing about the iPhone is that there are literally thousands of apps you can download to make your phone do more. Similarly, there are thousands of "extensions" you can get for Magento that add functionality to what your site can do. You can get software that integrates with your shipping providers, analyzes your sales data to find trends, changes the look of your site, and more.
Magento is the leader of the pack when it comes to eCommerce platforms. Learn why by checking out our answer to the question, Why Should I Launch My Website on Magento?