WorkSheet / Spreadsheet:
A Worksheet is a collection of Cells (Rows and Columns) which enabled users to input data, charts, tables, Images, etc.
Excel 2013 Worksheet Limitations:
Maximum Rows & Columns in Excel 2013 are 1048576 rows and 16384 columns which is equivalent to 17 Billion Cells
WorkBook:
A WorkBook is a collection of Worksheets (Eg: Sheet1, Sheet2, Sheet 3)
In Previous versions of Excel, default Workbook used to have 3 worksheets, but in Excel 2013, there is only one worksheet by default. You can create sheets as per your requirement.
You can delete, Add, Navigate, Rename and Hide Worksheets based on your requirement.