You can organize your releases with Tags and Pointers. Tags can be used to organize types of releases. You can, for example, tag your User Interace-focussed released with the tag ui. Any release of your product can have the same tag.
A Pointer is a special type of tag. As the name describes, it points to a specific release. It can only exist once for each product. As soon as you apply a pointer for a release, the parent product points to it. You can use pointers to achieve things like the latest tag for Docker images.
Channels are primarily used to describe the status of an release. You can restrict the access to a specific channel for dedicated subscriptions. Tags are loosely coupled. You can search for them but you can't apply any access restrictions.