What is NHibernate?

NHibernate is a port of Hibernate Core for Java to the .NET Framework. It handles persisting plain .NET objects to and from an underlying relational database. Given an XML description of your entities and relationships, NHibernate automatically generates SQL for loading and storing the objects. Optionally, you can describe your mapping metadata with attributes in your source code.
NHibernate supports transparent persistence, your object classes don't have to follow a restrictive programming model. Persistent classes do not need to implement any interface or inherit from a special base class. This makes it possible to design the business logic using plain .NET (CLR) objects and object-oriented idiom.
Originally being a port of Hibernate 2.1, the NHibernate API is very similar to that of Hibernate. All Hibernate knowledge and existing Hibernate documentation is therefore directly applicable to NHibernate. Take a look at the available NHibernate Documentation.
NHibernate 1.2 introduces many additional features from Hibernate 3. Details about the new features can be found in this blog post.
NHibernate key features:
Natural programming model - NHibernate supports natural OO idiom; inheritance, polymorphism, composition and the .NET collections framework, including generic collections.
Native .NET - NHibernate API uses .NET conventions and idioms
Support for fine-grained object models - a rich variety of mappings for collections and dependent objects
No build-time bytecode enhancement - there's no extra code generation or bytecode processing steps in your build procedure
The query options - NHibernate addresses both sides of the problem; not only how to get objects into the database, but also how to get them out again
Custom SQL - specify the exact SQL that NHibernate should use to persist your objects. Stored procedures are supported on Microsoft SQL Server.
Support for "conversations" - NHibernate supports long-lived persistence contexts, detach/reattach of objects, and takes care of optimistic locking automatically
Free/open source - NHibernate is licensed under the LGPL (Lesser GNU Public License)