From Oracle FAQ
PeopleCode is an proprietary object-oriented programming language used to add business logic to PeopleSoft applications.
PeopleCode is a visual language similar to Microsoft Access or Visual Basic. Since PeopleTools version 8.1, the coding syntax was changed to reflect the Visual Basic. This makes PeopleCode easier to use and learn for developers who already know Visual Basic.
PeopleCode can be placed in events on the field definition in each record. PeopleCode can also be placed into events on fields within pages and components. Each event is a place of action that launches PeopleCode when certain processes take place on a page.