What is the difference between a relationship and marriage?

The relationship between a man and a non-mahram woman is either through marriage, which Allah has made lawful and upon which He has ordained great benefits, or through unlawful means such as taking secret lovers (akhdan) and anything that leads to the forbidden, like intermingling and seclusion. The difference between the two is vast; the lawful is permitted, while the unlawful is prohibited, and committing it can lead to Allah's wrath and punishment.