The main purpose of NHibernate is described in the Wikipedia article as well:
NHibernate's primary feature is mapping from .NET classes to database tables (and from CLR data types to SQL data types). NHibernate also provides data query and retrieval facilities.
NHibernate generates the SQL commands and relieves the developer from manual data set handling and object conversion, keeping the application portable to most SQL databases, with database portability delivered at very little performance overhead.
