Data Validation helps you to improve data quality by preventing users from entering incorrect data. There are Standard Validation (field type, mandatory field, single field) and custom validation with validation rules. The latter are set by defining one or more error conditions and the associated error message.
Validation rules are executed when saving the record. If any data is invalid, the associated error message displays without saving the record.
The number of validation rules per object is limited, for example in the Enterprise Edition, it is limited to 100.
Example: In the “Percentage of discount” field, the amount entered must be between 5 and 25, otherwise it will be impossible to save and an error message will be displayed.