You can get to this part of the system by clicking on the Products link in the blue Admin Toolbar and then clicking New Product > Descriptions Tab. You can also quickly open any existing product and get to this tab by going to Products > Products, searching for your item, and then clicking on its title and then the Descriptions Tab.
You can enter descriptive text to help market your products in this area. The Short Description is convenient if you don't need anything more than the basic details of a product. In this area, you are limited to 256 characters, including spaces and punctuation. The reason for this is because in a Google search result, no more than that will ever display. We recommend entering your description, saving the product, and then reopening it to make sure the message is not truncated (as the one below is, if you compare it to the Long Description).
The Long Description allows you more freedom in the content you enter for the product. You can include additional images, links to other resources (like PDF spec sheets or manufacturer websites), specially formatted text, tables, and even videos if you like. For your more expensive products, it will pay to put in a complete description here, and this is one of the key ways you can set yourself above your competition.
The Brief Description Below Add-to-Cart Button is an area about 400-pixels wide that you can use for content if you like. It will be as tall as you need for whatever content you want to add, and you can add rich content just like in your Long Description.