"au moins" means at least; so for example: Il a au moins 20 ans: you are sur that he is 20 and certainly more tha 20. "du moins" we use it when when you are not pretty sure about what we are saying, so for example: Il a 20 ans, du moins c'est ce que je crois: You only think that he is 20, cause you are not really sure. tell me if you what other explainations :)

Au moins is used to say +# (+ number) - il fait au moins 20 degrés. Or to say he did it - "Il aura au moins réussi à faire ses devoirs" http://dictionnaire.reverso.net/francais-definition/au%20moins,%20du%20moins,%20%C3%A0%20tout%20le%20moins