ADO.NET introduces a tremendous range of new componentry for developers to learn. For starters, ADO.NET's connection objectsa.k.a. database providers or managed service providersfall into two main categories:
A provider for general data access through OLDB
A provider "optimized" for Microsoft's own SQL Server database
Beyond that, Microsoft is also supplying APIs for custom database providers. IBM is now taking advantage of that fact. Meanwhile, two database providers are already available for Oracle: one from Microsoft and one from Oracle itself.