Hey @Renu353! Well, it's good that you are exploring new things and not just following the mainstream way to learn Hyperledger Composer.
The answer to your question, Online Hyperledger Composer is an online Web Sandbox Create, Deploy, Edit and Test business networks online. All of this is done on your browser and you do not have to download or install any Hyperledger-composer related software on your system. You can find an example of Online Hyperledger Composer here.
Alternatively, when you download and install required software and Create, Deploy, Edit and Test business networks on your system, that is called Offline Hyperledger Composer.
Hope you understood the difference :-)