A GatherContent project is made up of a number of Items. These Items normally reflect pages on a website, or pieces of content in another type of project. Items can be given a hierarchy when you're creating them, and are all stored in the Content tab.
An Item has a template applied to it, and this template contains all the fields which you can enter content into. A structure and a template are the same thing - a collection of fields - a structure is applied to one Item, and a template to as many as you want.
As you're writing and editing your content, GatherContent automatically saves any changes you're making as a revision. You can restore these revisions if you need to restore content to a previous version.
You can leave inline comments on content in a rich text field, which you can notify others of, and start a conversation around any changes that might be needed. A person can be assigned to an Item. When this happens, they're effectively responsible for it, alongside anyone else assigned to the Item.