ASP.NET membership is designed to enable you to easily use a number of different membership providers for your ASP.NET applications. You can use the supplied membership providers that are included with the .NET Framework, or you can implement your own providers.
There are two primary reasons for creating a custom membership provider.
- You need to store membership information in a data source that is not supported by the membership providers included with the .NET Framework, such as a FoxPro database, an Oracle database, or other data sources.
- You need to manage membership information using a database schema that is different from the database schema used by the providers that ship with the .NET Framework. A common example of this would be membership data that already exists in a SQL Server database for a company or Web site._
Above from MSDN
Also, they provides a very detail sample to explain how to implement a customer membership in ASP.NET
Please click here to read the sample code