What is the difference between a gallery and a collection?

A Gallery is a group of files. A Collection is a group of Galleries (or other Collections). 

You can make as many Galleries and Collections as you want, and organize them in any way that’s useful for you, such as by project or date or subject. (Change your mind later? No problem. Just drag and drop them from one Collection to another.) 

For example, many sports teams create a parent Collection for every season, and then nest specific Galleries for each game underneath. This makes it easy to locate images and share entire Collections with their clients (known in Libris as Invited Users).