When you remove a field from a content type, the data it holds are permanently erased. You will be asked to confirm this action
You will have to manually update your Views, pathauto settings, etc... if needed.
Note : if the field is shared across several content types, removing it from one content type does not affect the data for the other content types.
Removing a group from a content type does not remove the fields it contains, and therefore erases no field data.