Define Data Validation
Data validation is a method to conditionally prevent records from being saved; it is separate from field level security and page layouts.
List common use cases for Data Validation
See â€œUseful validation formulasâ€. A common usage would be to prevent an opportunity from being saved if the close date is in the past.
Describe when Data Validation Rules are enforced
Data validations are enforced whenever a record is saved. This includes the data loader, mass transfer records, importing records, etc. Validation rules during lead conversion are only active if enabled. See â€œValidation Rule Considerationsâ€ for details.
Create Data Validation Rules
Validation rules are created per object, and make use of formulas. I would strongly recommend reading and trying a few from the list â€œUseful Validation Formulasâ€.