Answer:
The difference is that the IaaS is completely configurable as needed, while the SaaS is a fixed, ready-to-use model.
Explanation:
Both IaaS and SaaS are cloud storage structures, used by a set of computers connected to each other, chewing to the point of sharing the same processing, memory and disk.
The main difference between these two structures is system configurability. That's because the IaaS is completely configurable, being able to completely adapt to the users' demand, allowing the systems to be scaled, or reduced according to the amount of resource, or demand. SaaS, on the other hand, does not have this configurability, being a system already fixed and ready to be used when users already have a very clear purpose, with determined and controlled resources that focus on just one need, already studied and understood.
