Toggl Track allows you to group your data in various categories (projects, clients, tags, etc) within workspaces. It's worth pointing out that the "names" of these structures are totally arbitrary terms. Anything can be a client and anything can be a project. If you don't have a need for both clients and projects, just use projects. There is nothing preventing you from naming your project "Client McClient".
Data hierarchy in a workspace from top to bottom
Can be associated with multiple projects. Can't be directly associated with time entries.
The main organizational structure in Toggl Track. Can be associated with just one client. Can contain multiple time entries. Billable rates, estimates and other features are defined on the project level.
The object that contains the actual time information. Can be directly associated with tasks and project, but not clients (that connection is made through projects).
These exist on the same "level" as time entries. Tags are essentially a way to provide extra information about an individual time entry. Most often used to mark time as invoiced or billed.
For more insight into how you should structure your data, please read this guide on categorizing data in Toggl Track.
Need more help with Toggl Track? Feel free to contact our support team via chat or email.