vtiger CRM modules can be classified into the following types:
Modules in this category will create entity records in vtiger CRM. The module will provide Create view, Edit view, Detail view and List view. You will be able to create filters etc.
Entity modules are recommended for cases where a new type of data object, e.g. Timesheet, needs to be added into the system as part of the new module. These new data objects can be viewed and managed by administrators and users.
Leads, Contacts, Accounts, Payslip, etc... are Entity Modules.
Modules in this category need not follow the general behavior of the Entity Module. The records created by Entity module could be used to provide a extended functionality or the records creation/editing can be handled in its own way.
Extension modules can be used when add-on functionality is needed, without the need for new kinds of data objects that users view and manage.
Dashboard, Reports, Portal etc... are Extension Modules.
Language Packs for vtiger CRM are also treated as another kind of module by vtlib.