Cells are individual boxes in a spreadsheet. Identify a given cell by its column letter and row number (e.g.: C3).
Worksheets appear as tabs in an Excel file. Identify a cell within another worksheet by adding the worksheet name and exclamation mark before the cell identifier (e.g.: Sheet1!C3)
Workbooks are individual Excel files. Identify another workbook by adding the file path and file name (in brackets) before the worksheet and cell identifiers (e.g.: 'C:\Temp\[Book1.xlsx]Sheet1'!C3).