A GatherContent account is split into different Projects, which are effectively different buckets of content. Normally, Projects signify different sections of a website, or even completely different websites, that you're working on.
A Project is made up of a number of Items, which can be organised into a hierarchy to show a website's structure. This hierarchy can be reordered from the Content tab, or defined when creating new Items.
Projects also consist of Templates and a Workflow, where due dates can be applied to each step in the workflow.