In the case of IaaS the computing resource provided is specifically that of virtualised hardware, in other words, computing infrastructure.
The definition includes such offerings as virtual server space, network connections, bandwidth, IP addresses and load balancers.
Physically, the pool of hardware resource is pulled from a multitude of servers and networks usually distributed across numerous data centers, all of which the cloud provider is responsible for maintaining.
The client, on the other hand, is given access to the virtualised components in order to build their own IT platforms.