IaaS, Infrastructure as a Service, refers to cloud based services that are not full end user functional products of themselves. Google Drive is an SaaS, AWS Glacier is IaaS.
IaaS offers core components of your infrastructure to provide services ranging from Backup Storage Locations to Content Delivery Networks, or User Authentication Services.
Different IaaS vendors provide different products and specializations. There services can range from standard networking and storage, to vision recognition and even genome sequencing.