FluentValidation is a .NET library for building strongly-typed validation rules using a fluent interface. It supports both synchronous and asynchronous validation, localization, conditional validation, rulesets, and cascade modes. The architecture centers around AbstractValidator<T> and IValidator<T>, with extension methods for common validation scenarios. The library is designed for use in various .NET applications, including ASP.NET Core, and provides dependency injection integration. Key components include a high-performance accessor cache, message formatter for templating, condition builder for reusable predicates, property chain utility for object graph navigation, and validation strategy for configuring validation scope.