All internal users can manage their dashboards: add or remove dashlets and tabs, edit dashlet options.
Admin can edit dashboards of other users. On the user detail view click Preferences from the dropdown at the top-right corner. There you can find Dashboard Layout.
Users can reset thir dashboard layout to the default. On Preferences edit view click Reset Dashboard to Default from the dropdown next to the Edit button. Admin can reset dashboards of other users.
The default dashboard layout will be applied for all new users. It can be changed at Administration > User Interface > Dashboard Layout.
Available for admin at Administration > Dashboard Templates. Dashboard templates provide the ability to deploy a specific dashboard layout to multiple users. It's possible to deploy for all users of a specific team.
On the edit view of portal record you can define its default dashboard layout. It will be applied for all portal users unless a user has a specific Dashboard Template.
When editing Portal User you can specify a dashboard template for the user. This template will override the default template of the portal.