Situation: You are on machine A, and you want to log in machine B without having to enter your password each time.
On machine A:
type: ssh-keygen -t rsa
then press enter for default settings, and make sure you don't enter any paraphrase.
Now, you need to send your public key to machine B.
type: cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys'
That's it !