Galleries are virtual showcases you create of photos and videos from other Flickr members. Kind of like an physical photo gallery in a city, but on Flickr. Galleries are a great way to engage with other members of the Flickr community and to show them you appreciate their work.
What is the difference between a gallery and group?
A gallery is a curated set of images from Flickr that you control. You can add images to them (assuming members have selected the Setting that allows their work to be added to galleries), edit the order of those images, and remove them as you like. Groups are where many people add their own images based on a theme or concept. Photos have to be invited to groups or added by the photo owner, whereas gallery creators can add any photo to a gallery if permissions have been set. You can create a gallery by going to your Galleries list page.
Groups have admins, moderators, and members, with different controls for the different levels. Admins determine the theme of the group, the rules for adding images and participating in the groups, the conduct that is acceptable or not, etc. Moderators are able to remove images from groups and have slightly more limited roles.