Envato is an Australian-based company that runs several marketplaces. For WordPress users, the relevant destination is (themeforest.net), often colloquially referred to simply as “Envato” when discussing themes.
The (one end product, one domain) covers most uses. The Extended License (for products you resell or for SaaS) is much more expensive (often $1500+). Many beginners accidentally violate terms by using a Regular License on multiple client sites.
This is a marketplace where you buy individual theme licenses. It is known for its massive selection of over 18,000 templates and themes, often with dedicated six-month support periods included. Developers here are often more invested in individual product updates.