For several years now, the cloud has been at the heart of the (IT) news and has been fuelling the debate. But for ordinary people (except geeks - see article -, computer scientists and developers), it is often complicated to understand what the cloud really is. A very broad term that includes a whole bunch of definitions, concepts and notions. We will try to clarify all this with a simple and concise definition. Let's start at the beginning with the translation of the word "Cloud" which means "cloud" in English. Namely, by convention, when we schematize a network, the Internet is represented by a cloud (see example). In reality the cloud is a marketing term for everything online. In a few words we can define the cloud as "a concept that consists in deporting to remote servers, computer processing traditionally located on the user's client computer. "(Wikipedia) The cloud is characterized by the following elements: - Access from all devices connected to the Internet, i.e. from anywhere in the world. - Increased flexibility. The capacities of the infrastructure increase on demand, a flexibility that is theoretically infinite. Concretely, if your site is hosted in the cloud, the characteristics of the infrastructure will adapt according to the number of visitors. You won't need to ask for an increase in performance, so your host will not have to transfer all your data to a more powerful server. The cloud offers a real revolution in terms of elasticity. - A redundancy that ensures the security of your data, on the one hand for the conservation of your data, and on the other hand against hacking. This security is possible thanks to the permanent duplication of the process and data. - Due to self-service through the internet, the cloud allows a minimal management effort. Indeed, no installation or updates are necessary, and human interactions are reduced to a minimum. This revolutionary model is used as much by professionals (SMEs, large groups, IT professionals) as by individuals. As an example we can quote Google Drive, Chrome OS, Microsoft Online Service or the Cloud offers of OVH. In short, the cloud is the result of a natural evolution of the Internet which aims to put online a set of services, infrastructures and platforms. We hope we have kept our promise !