A data model is the way you describe the data you want to store. For example, a simple data model could be described in SQL as: CREATE TABLE EMAILS(name varchar(100), email_addr varchar(100) ).
A data source usually refers to a way to describe the exact database you are talking to. For instance, there may be multiple databases running on a machine, so in order to specify which database you want, you might use a string like "localhost:5432:patrick" to specify that the machine name is "localhost", the TCP/IP port to connect to is "5432" and the name of the database is "patrick".
Usually there are slight differences in the format of the data source depending on which database server (Postgres, Oracle, Sybase) you are using.
