Explanation/Reference:
* Multi-tenancy
Infrastructure as a Service is very similar to what we've known as hosting or collocation services, just painted with bright cloudy colors. Multi-tenant services are any services that you offer to multiple customers, or "tenants." In most cases, IaaS and multi-tenant services mean the same thing, although you could also implement storage- or database-related multi-tenant service.
* Hardware
In the case of IaaS the computing resource provided is specifically that of virtualised hardware, in other words, computing infrastructure.
Note: Infrastructure as a Service (IaaS) is a form of cloud computing that provides virtualized computing resources over the Internet.
References:
http://www.interoute.com/what-iaas
http://searchtelecom.techtarget.com/answer/How-do-Infrastructure-as-a-Service-and-multi-tenant-services- differ