MS EXCEL - Validation

Data Validation - It allows you to define validation on cells for entering value.
Common Validations are - List, Date, Time, Text Length etc.
List Validation - It will create a drop down containing a range or defined values

Select the Cell/Range to Validate -> Goto Data -> Data Validation -> select the Validation criteria here and press OK. Now cell will take only defined values

Circle Invalid Data - It will put circle on cell containing Invalid values, when validation is defined after entering values.