A public cloud is any service provider that provides hosting or Infrastructure as a Service (IaaS). Public clouds were spurred on by the emergence of virtualization Hypervisors. You can rent virtual machines and virtual servers in the cloud for a fraction of the cost of buying and maintaining your own equipment. As more and more companies are realizing the benefits of a public cloud, they are moving some resources to the cloud. One of the caveats, of course, is that you don't really know who you are sharing your infrastructure with. Some companies have been slow to move to the public cloud as they are apprehensive about its security and performance.