OLE DB is a COM based framework that allow to transfer (tabular) data from a data source to a consumer after the consumer has «asked» for it.
OLE DB is one of the primary MDAC technologies (the two others being ADO and ODBC).
OLE DB was/is the (COM) successor of ODBC: OLE DB provides a uniform set of COM interfaces to be used by different applications.
Contrary to ODBC, which only allows to access relational databases, OLE DB can also be used to access other data stores.
A so-called OLE DB connection string identifies the data source and several attributes such as username/password (for authentication) or time-out properties etc.
Usually, OLEDB is used to create database drivers which are then used by another database access layer such as ADO or ADO.NET. In such a constellation, OLEDB is called the provider and ADODB the consumer.
This is why an ADODB connection string might contain something like provider=Microsoft.Jet.OLEDB.4.0;.
Data source
An OLE DB data source identifies a database that is accessed through OLE DB.
In ADO, a connection to a data source is represented by an object whose type is ADODB.Connection.