Well there are two options really.
Two Applications
Have two applications totally seperate, this means they have nothing in common. Seperate login, seperate models, seperate libraries, etc. That may be useful, depending on what sort of backend you have, as you may have loads of huge big functions and libs that frontend users will never need, or you may not have the same login for both admins and users.
Admin Sub-Directory
If you will need to use lots of similar data/files between both the admin and frontend, it makes sense to just have the admin in a sub directory and use all the same stuff. I normally do it this way as my sites are often basic “user registration/forum/blog” type sites with a few extras. No need to seperate it.