What if the Spider Man joined the Fantastic Four? Or what would happen if Doctor Strange worked for the evil demon Dormammu? What events would unfold if the Hulk had Bruce's intelligence? Stan Lee and the rest of the Marvel talents dicided to answer that.
In What If...? (officially copyrighted as What If) vol. 1, Utau the Watcher (a cosmic being that lives on the moon) narrated what would have happen if events took a different turn then they did by showing the reader alternate realities. Such as, issue X might have "What if Blob joined the X-men?" or "What if Reed Richard became the Thing?". The series proved so popular Marvel created eight volumes of it!