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The term "cloud computing" http://www.webopedia.com/TERM/C/cloud_computing.html to a network architecture where various computers - not necessarily identical, or of equal capacity - collaborate in carrying out a certain task. Being that new computers can be freely introduced into the set, or removed from it, without the need to stop processing and/or reconfigure everything.Nowadays it is most used in the context of http://pt.wikipedia.org/wiki/Software_como_servi%C3%A7o *, where you do not perform computing on your computer(s), but rather hire someone to host the necessary systems - paying only for the processing power, disk space, network connections... And faithful to the philosophy of the cloud, allowing yourself to increase or decrease the size of the contract (i.e. how much you pay, and how much resource is allocated to you) in an easy way.*Or, how https://pt.stackoverflow.com/a/30308/215 , other types of service, but maintaining the characteristic of not "engest" the system to the environment being executed.Putting this in the context of your question:Both webmail services and other client-server applications (e.g. Viaweb - then Yahoo! Store - first web application of history) are not considered "cloud" because there is no such scaling feature: the provider offers a fixed and uncustomizable service, and a certain amount of resources, and only. Sometimes there are different plans (e.g. 1GB of email space, 5GB, etc.) but it is rare to see something "under-demand" (where you pay more or less, month by month, as your need forecast).The example of ERP is similar - the service is fixed, the conditions are determined by the owner of the company... Transfer to a third-party server, on its own, does not change anything (say that you http://pt.wikipedia.org/wiki/VPS - at a fixed cost with fixed resources - this does not characterize cloud computing). What can happen is that this third-party provider offers hosting with cloud computing features (you choose how much space you will need and how much monthly transfer, paid proportionally, a little more if you exceed the limit, etc).Again, the site would not be a cloud example. But if the site is hosted on a server that charges proportionally to the network traffic used, or the disk space used for blog/comments posts, then it would be a cloud service.As https://pt.stackoverflow.com/questions/30302/o-que-%C3%A9-computa%C3%A7%C3%A3o-nas-nuvens#comment56550_30302 , there is no simple criterion to say if something is "in the cloud" or not. Nowadays this expression is widely used as synonym of "on the web". But I would say that if the service does not allow to increase or decrease resource allocation without moving the settings [from the customer's point of view, the contractor], I would not call it "cloud" - even if it is described as such (and would not heat the head with it, after all a name is just a name...).Addendum: in my answer I always referred to "a service you hire", but in reality there is no need for a third-party contract for a platform to be considered "cloud": you may have a company with own infrastructure (servers, co-located or hosted your own dependencies) that organizes and makes available your computing resources in the form of cloud. Being that the various systems installed on the internal network, the various departments of the company, etc., have resources allocated to them dynamically, and non-statically (the X server hosts the application A, the Y applications B and C, etc; we will deploy the D service? we need to buy another server... etc.). It is what we would call "private cloud" ( http://en.wikipedia.org/wiki/Cloud_computing#Private_cloud ).