According to my knowledge, SaaS consists to Software as a Service model where companies do not need to make the purchase of licenses, installations of the product in each of the users PC, perform version upgrades, software corrections, etc.
Among others, since it takes is a right to use the software or pay a lease accessing this via the Internet. SaaS allows companies to keep off large investments when they start a project to systematize and then maintain high operating costs during such payment of maintenance personnel, etc..