Hi,
ODBC is Open DataBase Connectivity, it is a Microsoft standard to allow any application to access data stored in a remote system. It runs on the client system.
An ODBC driver is supplied by the vendor of the database, or other source of data (eg Oracle, Sybase, Microsoft etc) and will allow any application that can connect to an ODBC source to read/write data on it. It is important to note however that it doesn't have to be a database, there are ODBC drivers available for spreadsheet programs, text files etc as well.
An ODBC Data Source brings together a specific ODBC driver and settings to connect to a specific data source. DSN stands for Data Source Name - it is the name of the data source.
When you connect to an ODBC data source you connect to its name - the ODBC interface uses the data source settings and the driver to connect to the data source and read/write the data.
John