classified.abc.com/ but Its no display anything on front end,How I check whether Its subdomain or sub-directory,I forget which mode I selected
It appears you have a subdomain configuration. Otherwise it would be abc.com/classified.
As for it not showing anything on the front end...first off, does that mean it DOES show something on the back end? If that is the case then you have either a theme or plugin problem. If neither the front end or back end of the sub site are accessible than my first guess would be that you do not have wildcard subdomains configured in your host.
To setup wildcard subdomains you just need to go into your hosting account and add a subdomain that has * as its value and points to the same directory as your WordPress installation. This should do it as long as your host allows wildcard subdomains.
also My users are not showing in classified.abc.com, It only shows my account ,I want to use same users for all
Users are not automatically added to each subsite by default. You would normally have to manually add them by either editing the site within the network dashboard on the Users tab or by going to the dashboard of the subsite as an administrator and using the "Add existing user" option on Add New under users.
There are also plugins that can help with this. Join my multisite is pretty cool and has an auto add option. Multisite user management is also very useful for this kind of thing.