A WebObjects application will access persistent data via the JDBC protocol. See http://java.sun.com/j2se/1.4.2/docs/guide/jdbc/ for an example of the documentation published by Sun for JDBC.
When one deploys a WebObjects application, the EOModel used to access the data for the application will include the "jdbcURL". This will be used to tell EOF how to access the data. It is possible to specify the full name of the JDBC driver class, but commonly used databases have a default driver class that will often be correct. For example, one may use the MySQL database and specify that the driver class is "com.mysql.jdbc.Driver", but this is also the default value.
It is possible to use any data source, as long as there is a JDBC driver for it. For example, a driver could be created which will read data from a chat server and convert it to something that appears to have come from a database, which allows a WebObjects application to process it as it would any data.
There is documentation specific to various JDBC drivers published by the entties that created those drivers. Links to some of these references follow:
See references to the JavaJDBCAdaptor.framework for more information about how WebObjects uses JDBC.