From http://wiki.vtiger.com/archives

Workflow

Is a feature which will automate sending Emails , creating Calendar Event like To Do's and Event and also support Custom events based on the conditions given.

vtlib-Related Entities

Vtlib

vtlib is a library to ease new module development for vtigerCRM. vtlib includes APIs to create or modify the backend elements for a module. These APIs help make the necessary changes to the database.

Web Services

The vtigerCRM Web Services API provides a well-defined API for development of new plugins and integration of third-party applications. This will ease development of applications/plugins around vtigerCRM and simplify integration with external applications as well

Eventing

Eventing API provides a simple way of adding actions without making any modification to vtigerCRM.

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UI Types

Know the meaning of the UI Type numbers used in VtigerCRM development process.

Debug Techniques

Know the tips about debugging vtigerCRM during development phase.