1) - Difference between Admin and Catalog?
Admin is where you and the staff/partners login to manage the online store. Add categories, products, customers, orders (if you want to take order by phone for example). in short anything that has to do with store management.
The Catalog is the facade of your store, it is what a visitor to your site will see.
2)I can create user in the admin panel with module.But how to create 2 kind of users who can register/login in the front-end and can sell the item of the OpenCart Store.
I am not sure what you mean by module here. There are two type of users, an Admin user / and a Customer.
Admins are anyone who has access to the backend of your store and can performs tasks according to the level of privileges you grant them. if you want someone to be able to post product you must give him/her access to you admin, in other words, you must create an admin user.
A customer is just that, anyone visiting your site is a customer.
3) How can i configure it so that user can sell any item in front-end? How to fetch data for this?
For this you will need some major work. you will have to create the front end view for submission, with the controller and model that goes with it.
That said, you must consider how a user will be collecting the payment, open cart only allows for 1 payment gateway setting. So if you added your paypal account, that will be only account. But then again, you could work it out, but you will end up making a LOT of changes.
Opencart is easy to modify, the code is straight forward, although subject to debate on its quality, but it's opensource, you could build on it, or fork from it and create your own.
Cheers