When you add or edit a product you are presented with details its details. This data is split into tabs. The following explains each tab's purpose and links to relevant information.
This is the basic information about the product.
Here you choose which images are displayed for this product. The first image will be used on the category page and all the images used on the product page.
Metadata is Search Engine Optimisation information relating to the given product and will be used on its product page.
Here you choose which categories the product will be listed in.
Here you choose products that relate to this product. Depending on your website, there may be multiple types of related products which are displayed at different points on the product page.