Clouds: IaaS, PaaS, SaaS

We always hear: Go to Cloud! Go to Cloud!
Lets say you ready for this. Then, it will good to have a fast overview of the provided service models: IaaS, PaaS, SaaS.

IaaS
IaaS - Infrastructure as a Service. Infrastructure means a computer, in most cases - virtual machine.
Examples: Amazon, Rackspace, etc.
Positives:

1. No constrains on your platform/applications. You can install any Operation System: Windows, Linux. You can install any software: including you favorite database or frameworks Java, .Net, etc.
2. Hardware - it is problem of your service provider.

Negatives: 

1. Backups - it is your problem. Your virtual machine may fail, and lost all data (some workaround exist).
2. You need install and configure all software by your self (or have your own administrator).
3. Scaling - it is your problem.Yes, you can buy several virtual machines any time, but you have to configure them to work together and split payload, like load balancing.

PaaS
PaaS - Platform as a Service. Platform means - fully configured execution environment.
Examples: GAE, Amazon Elastic Beanstalk, Windows Azure, etc.
Positives:

1. Backups, software configuration, scaling - this is the problem of service provider. in most cases you will have user-friendly  management console.

Negatives: 

1. Constrains on your platform/applications. GAE supports mainly Java framework, Windows Azure - Asp.Net framework. Because of problems with Relation DBMS scaling  - most of providers encourage using of No-SQL databases.
2. Often, you need prepare(modify, rewrite) your application to be loaded into PaaS. Like, not all javaEE applications can be executed on GAE because it do not allow writing files on disk. Because of such contains, you may be faced with vendor lock-in, like, you can-not not simply migrate to other provider.

SaaS
SaaS - Software as a service. You may not buy, or write any software at all. You can just use and configure existing. Like, you can use web-based CRM or ERP and configure it to meet your needs, or use on-line website builder.
Examples: Google Apps, Salesforce, Wix, Blogger :), etc.
Positives:

1. You do need to write a software. You will have already tested software product.
2. Fast start.

 Negatives:

1. Vendor lock-in.
2. Modification of software in limited, like configuration only.