Hi Sun,
There are many SAP modules. Basic of them includes SD(Sales and Distribution), MM(Materials Management), FI ( Finance), HR (Human Resources) which is now called HCM (Human Capital Management).
Their names itself define their purpose. For example, FI is used to record in database all finance related transactions.
Other modules of SAP are ISU (Industry specific solutions like Gas supply, water supply, electricity supply), CRM(Customer Releations Management), SRM(Supplier Relationship Management), QM(Quality Management), etc.
Hope this helps you.
Regards,
Richa