The bane of spreadsheets and databases is GIGO - Garbage In, Garbage Out.  Having a way to verify data when it is entered, eliminates typo's and other data entry errors from becoming a part of your spreadsheet or database.  Here is how you can use Input Validation in Excel to attain that data entry protection.

This example will prevent someone mistakenly typing alpha characters ('cat') into a cell we have defined to accept positive integer values between 1-100.  Select a cell, and then from the Main Menu select Data/Validation, which will open the Data Validation menu.

You can enter titles and messages for the Input Message, and Error Alert tabs.

Now test the cell by entering anything except a positive integer between 1-100, and you should get your Error Alert message, and the data will not be entered.  Then try entering '42', and your data entry should be accepted.

Input Validation works with dates, times, text, words and formulas too.  Now you should be able to prevent data entry errors from creeping into your spreadsheet, and provide hints to users about the data you want entered.