Mark Rowlinson provides a discussion by Kid Van Ouytsel that does an excellent job explaining data validation. He has also constructs a sample workbook that you can download and play with.
"Data validation is a tool that can help you control the input/changes someone can make in a spreadsheet. It can help you or your users to make choices, guide them to make relevant input/changes, or restrict input to a specific type of data or structure. It can help you or your users to save time and to keep formulae working properly."
Excel -- Data Validation
Data Validation and Conditional Formatting
Microsoft Office Online:
Overview and samples of data validation
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