jQuery.validate is a JavaScript plugin that provides validation functionality for form inputs. It is built on top of jQuery and provides a set of validation rules that can be easily customized to meet the specific needs of your application. Some of the validation rules that can be defined include:

• Email address validation
• Password validation
• Uniqueness validation
• Length validation
• Charsets validation
• HTML tags validation

jQuery.validate is also capable of validating complex form structures and performing server-side validation when necessary. It is also easy to integrate with other jQuery plugins and libraries, making it a versatile and flexible solution for form validation.