Always means you do something regularly. For example:
I always brush my teeth, before going to bed.
Forever means till the end of time. For example:
I will love you forever (which means I will continue to love you till the end of time, for eternity etc.)
sometimes they can be used in each others place. For example you can say:
I will always love you
but you cant say
I will brush my teeth forever (because it means you will start brushing your teeth and continue doing it till the end of time, or till you die :) )