The default access level for classes is private, while for structs it is public.

The default inheritance type is private when deriving a class, and public when deriving a struct.
In other words, deriving a class from a struct or class is private by default, while deriving a struct from a class or struct is public by default.

Contrast the two examples below. In the first, a struct is derived from a class (public inheritance).

#include <iostream>

class C
{
public:
    void doStuff()
    {
        std::cout << "Doing stuff" << "\n";
    }
};

struct S : C // This is public inheritance
{
};

int main()
{
    S s;
    s.doStuff();
}

In the second a class is derived from a struct (private inheritance).

#include <iostream>

struct S
{
    void doStuff()
    {
        std::cout << "Doing stuff" << "\n";
    }
};

class C : S // This is private inheritance
{
};

int main()
{
    C c;
    c.doStuff();
}

The first example will compile, while the second produces the following error with g++:

class_vs_struct.cpp: In function ‘int main()’:
class_vs_struct.cpp:5:10: error: ‘void S::doStuff()’ is inaccessible
     void doStuff()
          ^
class_vs_struct.cpp:18:15: error: within this context
     c.doStuff();
               ^
class_vs_struct.cpp:18:15: error: ‘S’ is not an accessible base of ‘C’