What is a null-terminated string?

How does it differ from std::string?

A "string" is really just an array of chars; a null-terminated string is one where a null character '\0' marks the end of the string (not necessarily the end of the array). All strings in code (delimited by double quotes "") are automatically null-terminated by the compiler.

So for example, "hi" is the same as {'h', 'i', '\0'}.

A null-terminated string is a contiguous sequence of characters, the last one of which has the binary bit pattern all zeros. I'm not sure what you mean by a "usual string", but if you mean std::string, then a std::string is not required (until C++11) to be contiguous, and is not required to have a terminator. Also, a std::string's string data is always allocated and managed by the std::string object that contains it; for a null-terminated string, there is no such container, and you typically refer to and manage such strings using bare pointers.

All of this should really be covered in any decent C++ text book - I recommend getting hold of Accelerated C++, one of the best of them.