Nós is the pronoun we,in English.
Nos is an oblique pronoun,that can be used as an object,referring to nós,like:
Ajude-nos a aprender inglês.(Help us to learn English).
But there are also exceptions,like:
The lesson was made by us. (A lição foi feita por nós). Note that,in this case,you don't use nos,because there isn't a verb together with the pronoun.