REALBasic provides a number of options for the Database your program can use.
The simplest is using the built in Database, which produces a single SQLite format file. The main disadvantage with this is only a single program can safely access this database at any one time.
For a database that multiple users can access simulataneously, you need to go on to a client/server database system. The three main systems supported are REALServer, MySQL and PostgreSQL. It is also possible to connect to various other databases using ODBC.
Waiting for someone to write the rest of this...