On Prem – Everything is hosted in customer’s datacenter
Infrastructure as a Service : IaaS
Microsoft provides basic infrastructure, like VMs/ Network But customer will have full control over what OS/apps they want to install, Customer Pays only for Storage/Networking charges etc.
Platform as a Service : PaaS
Microsoft provides basic infra + managed OS on VMs, Shared Network Controls , Applications which can be partially configured by user + partially managed by microsoft, AD
Software as a Service : SaaS
Commonly User, Almost everything on platter by Microsoft (managed wordpress, office 365 ), Users control access, and just use applications as per thier needs without requiring any setup etc from their side.