Marriott recently completed a pilot, in which it rented 340 properties in Paris, Rome, Lisbon and London. The pilot is expected to serve as a model for a US-based, home-rental program. Though, we don't know when or where that will launch.

With about 1.3 million guest rooms globally, Marriott is the world's largest hotel operator. But Airbnb, the world's largest home-rental platform, boasts as many as 6 million rentals globally. While Airbnb has faced its fair share of challenges -- from privacy concerns to lawsuits over ads and data -- those close to Marriott say the home-rental market is too big an opportunity to pass up.

Airbnb is expected to go public no later than 2020, and in the meantime, it's been busy diversifying. There are rumors of original TV content meant to spark the travel bug. It's toyed with the idea of building houses, and it has framed itself as a way to make corporate meetings more fun. While Airbnb is no longer strictly home rentals, that is still the company's bread and butter, and time will tell if Marriott is able to cut into Airbnb's main business.