When you are adding a product, you will be able to include a Product Description (Full Text) or Product Description (Short Text).
If you are on your homepage or your categories page, the short text will be displayed if you press the Quick Shop button.
Short Description will appear when you hit 'Quick Shop' - as below:
The Product Description (Full Text) will be displayed on your product page. Select 'View Details' and you will be taken to your product page where your long description will appear.