Here’s a 5-minute video showing what happens when a laptop battery ignites: It sets off a chain reaction, where successive battery cells ignite, each one releasing more heat and increasingly big jets of flame (up to 6 feet high). Amazing and scary.
As the narrator points out, you need to be careful with lithium-ion battery fires, since they’re electrical fires, chemical fires, and metal fires all wrapped up into one hot little package. Trying to put out the fire with the wrong substance (like water) could just fuel the fire, or spread it around. The video’s producers recommend a Class D fire extinguisher.