This chapter will show you more details about writing common OTRS Modules.
Core Modules
Core modules are located under $OTRS_HOME/Kernel/System/.
There you will have
Log (Kernel::System::Log),
Ticket (Kernel::System::Ticket),
Auth (Kernel::System::Auth),
User (Kernel::System::User),
Email (Kernel::System::Email), ... core modules for OTRS.