Not really relevant, as you probably will never encounter it.
Sheets collection contains all types of sheets. Although today we only use 2 types of Sheets, Worksheets and Chart Sheets, there used to be 3 more types of Sheets, like Dialog Sheets or Macro Sheets. You may still have in your company old excel files that use them. In that case if you check the Sheets collection you'll seem them all there.
In conclusion
Sheets - collection of the Sheets of all types
Worksheets - collection of Sheets of type Worksheet
Charts - collection of Sheets of type Chart Sheet