IaaS is a Cloud Service Model, stands for Infrastructure as a Service. Lets try to understand IaaS with the help of real world example, A house owner wants to keep his house on rent and for which he is finding some person who can live in that house on rent. Lets say he finds one person who is willing to take that house and live on rent. Now just observe the situation in this example carefully, here the house owner is going to give only the infrastructure to that person(client) on rent, means No furniture, television, utensils, appliances. Everything will be decided by the client, means TV should be there or not, what should be the paint color, should the furniture be of metal or wood.
Now here the house owner is going to be cloud provider and that person living on rent is a client or customer who has been given a infrastructure model. Now its the clients choice what should be there in hall, kitchen, bedroom and everything. So one point we can conclude here that the degree of freedom is more in IaaS as compared to other cloud service models. In IaaS we can choose which Operating system to work on. IaaS also provides virtual machines and storage areas, Also IP addresses are also provided. Now as per the client the resources can be scaled up or scaled down as a result of advanced scalability.
To get a IaaS practical experience you can login to AWS(Amazon Web Services) then go to compute and select EC2. EC2 is a good example of IaaS it provides scalable infrastructure for clients who want to host cloud-based applications.