Cloud Computing Basics

Hi everyone,
I have understand something about cloud computing and wants to share with you.

Cloud Computing mainly needed for the following 3 services
1. SAAS (Software As A Service): Where users can use the online-software tools
    ex: Google docs, zoho, pixlr, jaycut, aviry, netsuite, salesforce
2. PAAS (Platform As A Service): Where users can maintains online app development
    ex: Google App Engine, MS Azure, SalesForce
3. IAAS(Infrastructure As A Service): Where users can lease the infrastructure
    ex: Amazon EC(Elastic Compute)2, rackspace, gogrid

Correct if I am wrong in with my above explanation.
References:
1. Explaining Computing
2. Youtube

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