What are the characteristics of a SaaS application?

In SaaS, the user does not pay for the software itself. Instead, it works like a rental. They have the authorization to use it for a period of time and pay for the software that they are using. 

A great way to think about SaaS is like going to a public park. You go to the park to enjoy a picnic, play with your dog, have a pickup Frisbee game, etc. You have friends that also use the park. You might use it at the same time, or different times. You get to enjoy the park when you want to and as often as you would like, without having to mow the lawn, water the flowers and maintain the jungle gym. Instead, you pay a fixed amount of taxes per month and for that, the park is available to you anytime. The maintenance is taken care of for you, and local police patrol the area, keeping you and your family safe. 

A “park” meets the key characteristics of the SaaS model.