An image in a cloud server is a snapshot of a virtual machine, which includes a complete copy of the operating system and applications used to create and deploy virtual machine instances. In cloud computing, there are two common types of images: system images and application images.

　　System Image:

　　A system image is a complete image containing an operating system (for example, Ubuntu, Windows Server, etc.). It is usually a snapshot of an unconfigured operating system and only includes a basic installation of the operating system.

　　Using system images, you can quickly create new virtual machine instances and select the desired operating system version. You can then configure and customize it to your needs, installing the apps and services you need.

　　System images are usually provided by cloud service providers and include updates and maintenance to ensure the security and performance of the operating system.

　　Application Image:

　　An application image is an image that contains a specific application or application stack. It includes the operating system and pre-installed, configured and optimized applications.

　　Using application images, you can quickly deploy virtual machine instances containing specific applications without having to manually install and configure applications. This is useful for quickly scaling application deployments.

　　Application images are typically created by cloud service providers, developers, or administrators and customized to specific needs.

　　To summarize, system images are used to create basic virtual machine instances, while application images are used to contain virtual machine instances of specific applications. Usually, cloud service providers provide a series of commonly used system images, and users can create and manage customized application images according to their needs. The use of these images can greatly simplify the deployment and management process of virtual machines and improve efficiency and scalability in cloud computing environments.