OpenStack Image (Glance) : If we take example of simple hyperwiser like Virtual box. if we install one OS and install some software on top of it like web server, next time if you want to install the same OS and software you need to follow the same steps. Glance is a operating system image repository where you can store the Image as a template so next time you can use that template so installation of OS and other steps will not be required.
Glance provides discovery, registration, and delivery services for disk and server images. Stored images can be used as a template. It can also be used to store and catalog an unlimited number of backups. The Image Service can store disk and server images in a variety of back-ends, including Swift. The Image Service API provides a standard REST interface for querying information about disk images and lets clients stream the images to new servers.