When something is afire, it's actively burning with flames. A house afire is one that's actually on fire, with flames spreading through its rooms. A forest afire means trees and brush are burning, smoke rising into the sky.

The word creates a more dramatic picture than simply saying “on fire.” When a writer describes a building afire, you can almost see the flames leaping and hear the crackling. Historical accounts might describe whole neighborhoods afire during disasters, or ships afire during naval battles.

People also use afire to describe intense enthusiasm or passion. Someone whose imagination is afire has creative ideas bursting forth like flames. A speaker whose words set an audience afire has inspired them with powerful excitement. When your heart is afire with curiosity about something, you feel that burning desire to learn more.

It's more poetic and formal than “on fire,” which is why you'll find it in literature, historical writing, or when someone wants to emphasize the intensity of flames or feelings.