If you have a T* (called a pointer to an object of type T) and want to get a T (an object of type T), you can use the operator *. It returns the object pointed by your pointer.
In this case you've got a pointer to an object of type char* (that's it: (char*)*) so you can use *.
Another way could be that of using operator [], the one you use to access the arrays. *s is equal to s[0], while s[n] equals *(s+n).
If your char** s is an array of char*, by using printf( "%s", *str ) you'll print the first one only. In this case it's probably easier to read if you use []:
for( i = 0; i < N; ++ i ) print( "%s\n", str[i] );
Althought it's semantically equivalent to:
for( i = 0; i < N; ++ i ) print( "%s\n", *(str+i) );