If you've got some spare rooms in your house or extra property, you could traditionally rent it out. But then there's the problem of screening tenants. And finding someone long-term to have a business relationships with is an annoying and oftentimes scary endeavor.
And then Airbnb came along, which helped people with property have access to an entirely new income stream, especially if you live next to a poppin' area. You could make some serious coin if you price your place right and are a good host.
The first time I used the service, I have to say I was a little scared. I didn't know what my host was going to be like and if I really felt comfortable living in a room in someone else's home. But it ended up being super friggin' awesome. My host knew all of the best spots to visit in Miami (away from the tourist traps) and he helped make my vacation there with my wife an unforgettable experience.