The dbCreate property of the DataSource definition is important as it dictates what Grails should do at runtime with regards to automatically generating the database tables from GORM classes. The options are:

Both create-drop and create will destroy all existing data hence use with caution!

In development mode dbCreate is by default set to "create-drop":

dataSource {
   dbCreate = "create-drop" // one of 'create', 'create-drop','update'
}

What this does is automatically drop and re-create the database tables on each restart of the application. Obviously this may not be what you want in production.

Although Grails does not currently support Rails-style Migrations out of the box, there are currently three plugins that provide similar capabilities to Grails: Autobase (http://wiki.github.com/RobertFischer/autobase), The LiquiBase plugin and the DbMigrate plugin both of which are available via the grails list-plugins command