These are not industry definitions – just how I view things – I have been known to get things wrong, so don’t be running around preaching these as gospel….
IAAS: Infrastructure as a Service
Litmus test: you do not own the infrastructure. You do not have CAPEX. Its a service that you buy from a vendor and you have a monthly/yearly bill as OPEX.
You do not upgrade IAAS, you do not care what manufacturer’s chip is in your IAAS server – actually, you don’t care about individual server hardware.
Who qualifies? Not really anyone – not even AWS, after some bozo there decided to offer ‘Intel Inside’ instances. Its like a compiler requiring you to use bit of assembly code to get things done. Remember that C used to support it, and had created horrible platform dependent code issues. Yeah, AWS is making an abstraction mistake and they may pay dearly for this mistake.
May be Savvis does with Tier 3 addition, but I haven’t digged into it more.
PAAS – Platform As a Service
Litmus Test: A platform that developers use to develop, test, deploy, run applications – developers or ops do not manage this PAAS instances – you do not own servers, you do not buy VM software, you do not worry about infrastructure.
Who qualifies: SFDC almost does, but this SuperPod kind of makes them look dumb. Microsoft qualifies, but there are doubts on how much on-prem dependency they have. OpenShift online qualifies, CloudFoundry in a hosted manner qualifies.
Few more for good measure:
Private Cloud (n): A pink colored unicorn that spits rainbows.
SuperPod(n): A specially manufactured medical ventilator manufactured by SFDC to keep HP hardware business stay in business. Its strictly done to keep the patient’s family happy, fully knowing patient is terminal.
Single Tenant Multi-tenancy (v): Process of losing your brain cells because you hangout too much with new-age miracle-ists.