A site is a group of workspaces, potentially with custom branding, and potentially deployed at a separate domain name.
A single customer may have multiple sites - for example, which might correspond to international offices, or separate brands. However, most customers have a single site.
Internal and external users can be a member of a site, and each user could be a member of more than one site.
If a user has access to more than one site, they will have the option to switch between the sites within the user interface.