Zoho CRM comes with standard modules for Sales, Marketing, Customer Support and Inventory management etc. However, module requirements can vary widely from business to business. Custom modules give you the flexibility to create the niche solutions that maximize operational efficiency. With the Custom Modules functionality in Zoho CRM, you can develop new modules using built-in tools that need no programming skills. These custom modules can seamlessly integrate with core CRM modules and need not be stand-alone modules.
Check this video from Jeremy Nagel (a Zoho MVP) from Ethical Technology on creating custom modules.
Note: This video was created a few years back when the Zoho CRM UI was different. You may see screens differently today, but functionality is the same.