The "scriptwriter"is the person who has written all the dialogue for whats going to be said throughout the movie.
The "screenwriter"has the job of adapting the script to the needs of the director and producer and so,in effect re-writes the script so that it flows on-screen this can and,sometimes does result in the whole script being re-written.
Its a bit like editing to fit in with the length of the movie.
Quite often you will see where the credits are,story written by.......?