The primary difference between public and private clouds is the level of accessibility and ownership. A public cloud is owned and operated by third-party providers, offering services over the internet to multiple clients on a shared infrastructure, which allows for cost efficiency and scalability. In contrast, a private cloud is dedicated solely to one organization, allowing for greater security, customization, and control over resources, but often at a higher cost. Organizations may choose either model based on their specific needs, risk tolerance, and compliance requirements.