Workspace is a shared space comprising groups, folders and most importantly boards. Workspace members can be both Editors and Viewers and all members of the workspace can collaborate on boards stored in the workspace.
A team is essentially a secret workspace within a workspace. Any workspace members (except for Viewers) can create teams and then invite other workspace members to join. Only members added to the team will be able to view the boards stored in the team. Other workspace members won't see team boards and won't even know that the team itself exists.
Here's how the different sections look:
Think about workspaces as companies and teams as departments within these companies.