You gotta read the book, but some points of difference:

Lucy Mancini has a large part in the book but a bit part in the movie.

Fredo is a totally different character in the book. He's big and well-built (but still stupid). In the movie hes a bumbling idiot.

In the movie, as tough as they try to make Luca Brasi out to be, he still gives the impression that he has a soft side. In the book hes pure evil. I won't spoil it for you but I'll just tell you three words: baby, towel, ax.

Fontaine is a major character and one of the most interesting, multi-layered characters in the book. In the movie he has a bit part and comes off like a pompous fool. No wonder Sinatra got pissed at Coppolla.

No Dr. Jules Segal in the movie.

No Nino Valenti in the movie.

Also in the book, Wolz is some kind of child molester. A reference to this is made in one of the cuts that ended up in the Godfather saga, but never made it into the first-cut movie.

The book discusses the Corleone's relationship with the Cappone family out of Chicago, but not in the movie.

Simply put, you just gotta read the book.