Viewers are safe pairs of eyes. Leo, invited as a viewer, can browse Emma's comics, open any item, read a copy's history, and admire the statistics. He cannot add, change, or delete anything. Viewers suit family members who want to look things up, or club members who should see the shared catalogue without editing rights.
Editors are collectors at work. Sam, invited as an editor, can do everything involved in cataloguing: create collections, add items and copies, record history, tag, file, and organize. Editors suit anyone actively maintaining the catalogue.
Owners are also administrators. On top of everything an editor can do, owners manage the people and the account itself. Every account needs at least one, and KolleK will not let the last owner be demoted or removed.
Note
These roles apply to your whole account, not to individual collections. A collection's own visibility setting records who it is meant for, and once sharing is enforced it will decide who can see it, while roles keep deciding who can change it. See visibility and sharing.