Hi Abhishek,

Custom Portal:- A Salesforce Customer Portal provides an online support channel for your customers—allowing them to resolve their inquiries without contacting a customer service representative. With a Customer Portal, you can customize and deliver a visually stunning user interface to your customers, and use the following Salesforce features to help you and your customers succeed.

Use Of Custom Portal:The Salesforce customer portal is generally geared toward providing you a way to allow your customer's to see information that relates only to their account with you; things like their Account and Contact information, maybe the contracts they have on file with you, their Assets, etc. The primary limitation of a customer portal is that the portal user can only view their own account information.

Communities are branded spaces for your employees, customers, and partners to connect. You can customize and create communities to meet your business needs, then transition seamlessly between them.
Available in: Salesforce Classic and Lightning Experience
Available in: Enterprise, Performance, Unlimited, and Developer Editions

You can use Communities to:
Drive more sales by connecting your employees with your distributors, resellers, and suppliers
Deliver world-class service by giving your customers one place to get answers
Manage social listening, content, engagement, and workflow all in one place
Communities can be based on standard Salesforce functionality and tabs, or on one of our preconfigured templates. Communities can be used to share a subset of features and data from your internal Salesforce org. Communities can also be customized to use your company branding.
Communities live inside your org and can be easily accessed from the global header using the drop-down menu in the top left corner of Salesforce or in the Salesforce1 Mobile Browser App. Use this menu to switch between your communities and your internal Salesforce org.

For more information please check with below links:
http://www.kineticgrowth.com/salesforce-communities-customer-partner-portals/
http://salesforce.stackexchange.com/questions/15846/whats-the-difference-between-portal-and-newly-introduced-salesforce-community
http://help.salesforce.com/help/pdfs/en/salesforce_portal_to_community_migration_cheatsheet.pdfhttp://help.salesforce.com/help/pdfs/en/salesforce_portal_to_community_migration_cheatsheet.pdf
Hope this helps.

Please mark it as solved if it's resolved.

Best Regards,
Nagendra.