1) as said in Woocommerce documentation here http://docs.woothemes.com/document/related-products-up-sells-and-cross-sells/ :
related products is a section that pulls other products from your store that share the same tags or categories as the current product. These products can not be specified in the admin, but can be influenced by grouping similar products in the same category or by using the same tags.
2) What plugin have you bought?
As mentioned, related product are usually grouped by product category or tag.