Most fundamentally, the prepositions dans and en both mean "in." However, they are used differently and are not interchangeable. Describe some basic differences in usage.
Dans expresses an amount of time before something will happen: Nous mangeons dans deux minutes -- "We're eating in two minutes." It also means "in" when used with an article and a noun: dans la maison -- "in the house."
En indicates how long something takes: Je peux faire mes devoirs en trois minutes -- "I can do my homework in three minutes." It also means "in" when used with a noun without an article: en classe -- "in class/school."