There are two different types of products: "Simple" & "Variable"
Simple Products
Simple products are exactly as they sound, a basic product with one type/color/size/etc. They can be unique, or made in batches. An example of a simple product would be a one-size-fits-all hat.
Variable Products
Variable Products are products with several different versions. These can be different colors, sizes, patterns, etc. An example would be a T-Shirt available in Small, Medium, & Large