Here are some of the models of cloud computing: A public cloud is one based on the standard cloud computing model, in which a service provider makes resources, such as applications and storage, available to the general public over the Internet. Public cloud services may be free or offered on a pay-per-usage model. Community cloud shares infrastructure between several organizations from a specific community with common concerns (security, compliance, jurisdiction, etc.), whether managed internally or by a third-party and hosted internally or externally. The costs are spread over fewer users than a public cloud (but more than a private cloud), so only some of the benefits of cloud computing are realized. Hybrid cloud is a composition of two or more clouds (private, community, or public) that remain unique entities but are bound together, offering the benefits of multiple deployment models. It can also be defined as multiple cloud systems that are connected in a way that allows programs and data to be moved easily from one deployment system to another. Private cloud is infrastructure operated solely for a single organization, whether managed internally or by a third-party and hosted internally or externally. They have attracted criticism because users still have to buy, build, and manage them and thus do not benefit from lower up-front capital costs and less hands-on management, essentially [lacking] the economic model that makes cloud computing such an intriguing concept 2. Security concerns. Experts claim that their clouds are 100% secure – but it will not be their head on the block when things go awry. Its often stated that cloud computing security is better than most enterprises. Also, how do you decide which data to handle in the cloud and which to keep to internal systems – once decided keeping it secure could well be a full-time task.