Cloud – “A model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (for example, networks, servers, storage, applications and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction”
US National Institute of Standards and Technology

Cloud is not so much a technology as an approach to using technology. It provides massive IT related facilities through the internet, allowing customers to expand and reduce their storage space, data requirements and services as needed.
Public Cloud is a service that anybody (with internet and means of payment) can access. They are a shared infrastructure on a pay-as-you-use system.
Private Cloud uses a similar delivery mode to public cloud, but only for a select group of users, like the staff of a company. It can be considered much safer as the private cloud is behind a firewall and the companies own security systems.
Hybrid cloud (public and private that are linked) also exist, as do community clouds, a system shared by several group but not the public in general.

What are Cloud prices like?
You can store data on the Cloud with fast access time for about 25 cents per gigabyte for one month, or as an archived file for about 1 cent per month, archived files have slower access, but sometime only by a few seconds. You pay to upload and download information to those files.
Services on the Cloud can cost anything from 10 cents per hour to more than a dollar per hour.
How is software licencing affected by the Cloud?
This is a good question, whose answer is still in the process of being worked out. Previously, before the Cloud, a customer paid retail price for the software, even if they hardly used its potential capabilities. Cloud is potentially more flexible, and should allow individual and companies to only pay for services as they are used, and pay less if they only use a small part of the service’s potential. At the moment this is still being worked out. The software and its licencing are less flexible than what the cloud is capable of.
What about downtime and access of data?
Many cloud servers guarantee between 99.9% and 99.95% uptime, calculated over a year. This means no more than 4 to 8 hours of downtime (offline with no data access) in a 12 month period. But fine print in contracts sometimes means more downtime can occur that the provider will not take responsibility for. Really, there are no complete guarantees. But as even non-Cloud computers and other internet services occasionally don’t work, so we cannot expect perfection.
Will my application perform differently on Cloud?
Quite probably, but it depends on which cloud provider you have. Find one that works for you and runs services that are appropriate for your company. You may have to ask a lot of questions here and do a lot of reading, but there are appropriate providers out there for you somewhere. It helps to think of the situation as offering a lot of different options, and to think of cloud as upgrading your business operation. As such, you don’t have to follow the same procedure as you had before; look for a Cloud provider that offers a new and improved situation.
If you have Infrastructure as a Service (IaaS) or Platform as a Service (PaaS) you may have a lot flexibility over what applications you wish to run. However, the method of charging for this may vary. With Software as a Service (SaaS) you might only pay for software/applications as they are used; with PaaS you probably have to pay the full software price upfront.