What is cloud computing?

The term "cloud computing" originally referred to a network architecture where multiple computers - not necessarily identical, or of equal capacity - collaborate in the accomplishment of a certain task. As new computers can be freely inserted into the set, or removed from it, without the need to stop processing and / or reconfigure everything.

Nowadays it is most often used in the context of Software as a Service (SaaS) , where you do not perform computing on your computer (s), but rather hire someone to host the systems you need - paying only for processing power, disk space, network connections ... And true to the philosophy of cloud, allowing you to increase or decrease the size of the contract (ie how much you pay, and how much is allocated to you).

  

* Or, as pointed out by Caputo , other types of service, but keeping the feature of not " "the system to the environment being run.

Putting this in the context of your question:

• Both webmail services and other client-server applications (eg Viaweb - then Yahoo! Store - the first web application in history) are not considered "cloud" because there is no such feature of scheduling: Provider offers a fixed and little customizable service, and a certain amount of resources, and only. Sometimes there are different plans (eg 1GB of email space, 5GB, etc.) but it is rare to see something "on demand" (where you pay more or less, month by month, according to your forecast of need).

• 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, by itself, does not change anything (say you hired a < a fixed cost fixed resource - this does not characterize the cloud computing). What can happen is that this third-party provider offers hosting with features of cloud computing (you choose how much space you will need and how much monthly transfer, proportionally paid, a little extra if you exceed the limit, etc.).

• Again, the site would not be an example of a cloud. But if the site is hosted on a server that charges in proportion to the network traffic used, or the disk space used for the blog / comment posts, then it would be a cloud service.

As per comment of bfavaretto , there is no simple criterion to say whether something is "in the cloud" or not. Nowadays this expression is much used as a synonym for "on the web". But I would say that if the service does not allow you to increase or decrease the resource allocation without having to tweak the settings, from the customer's point of view, the contractor, I would not call it "cloud" - yet that is described as such (and would not heat up with that, after all a name is just a name ...).

Addendum: In my reply I always referred to "a service you hire", but in reality you do not need a contract with a third party for a platform to be considered "cloud": you can have a a company with its own infrastructure (servers, co-located or hosted on its own premises) that organizes and makes available its computing resources in the form of a cloud. Since the various systems installed in the internal network, the various departments of the company, etc., have resources allocated to them in a dynamic, non-static way (server X hosts application A, applications Y and C, etc. deploy service D? we need to buy another server ... etc). This is what we would call "private cloud" ( private cloud ). >     

According to the WikiPedia

  

Cloud Computing is the delivery of computing as a service rather than   as a product, whereby shared resources, software and   information is provided to computers and other devices such as   utility (such as the electricity grid) through a network (usually the   Internet) Clouds can be classified as public, private or   hybrids.

So CloudComputing is not the product delivered but the availability of services over a network.

These service models can be (according to the same Wikipedia article) :

Infrastructure as a service (Infrastructure as a service (IaaS))

Iaas providers offer computers (physical or virtual machines) and other resources, load balance, fail over, ability to scale services more or less depending on customer demand or need, storage with distributed backups, VLANs, firewalls.

Platform as a service (Platform as a service (PaaS))

As an example we have Microsoft Azure, Google App Engine, which scale processing and storage without requiring manual intervention of the developer.

Software as a Service (Software as a service (SaaS))

In this business model users have access to applications and their databases. Cloud providers provide the IaaS and PaaS that run the application. SaaS are generally considered "on-demand software" because they are paid per use.

In the SaaS model providers install and manage applications and software users access their services and pay by period or by consumption.

Thus:

  

One of the first applications that came out completely online is the   emails, like today we have Gmail, Outlook, etc., fully accessible   via browser. I've never seen anyone call this a "cloud", but have they not   would be the case, since this works without having to install   anything on the machine?

Install is not the point, email can be considered a SaaS and yes, it would be a Cloud because it is an application sold for you to use under the responsibility of another company

  

If a business owner resolves to release external access to the ERP in their   server, so that some people can access from their homes, this   already characterized as a cloud? If not, what would it take for this?   be a "cloud", put this on a third-party server?

No, it would be naked if the company's business was the ERP and it would sell access to it to other companies.

  

Could a simple website, such as a blog, be considered a mini-application in the clouds?

No, because it does not provide any service to your users

In your examples, I consider them as serviços and each service provider chooses how to deliver them (in which datacenter will host, how will the software, etc.)

computação na nuvem is when you run your app on a computer that you have contracted as a service and you do not even see it.

But only this description does not characterize cloud, since you need this service that is supporting the sua application to support some properties, such as scale on demand or aggregate services like firewall, routers, new disks, bandwidth adjustable.

I consider the best current example of Cloud Computing the separate services that are offered by Amazon AWS