NHibernate is an ORM (Object Relational Mapper). Its purpose is to map objects in your OO application to tables in a database for persistence.
Why would you need it? Because it can save you from writing a lot of tedious ADO.NET code. Essentially it enhances developer productivity when developing CRUD applications, that is, applications whose main purpose is to Create, Read, Update, and Delete data in a database.
NHibernate is open source, and you need to realise that you are making your application dependent on third party libraries, whose long term goals may diverge from yours.
If you want the productivity of an ORM without introducing this dependency, consider Entity Framework, or Linq2SQL (noting that Linq2SQL locks you into SQL Server).
And finally if you don't need the productivity enhancement of an ORM, and you want absolute control, stick to plain old ADO.NET.
