This is the study of biological anthropology which I'm quite shocked to see someone your age ask this splendid question ;)
The main difference that distinguishes homo erectus from homo habilis is the CRANIAL CAPACITY.
The cranial capacity of homo habilis ranged from 360cm³ to 600cm³
Homo erectus had an average cranial capacity of up to 1100cm³
And we, the homo sapiens have an average cranial capacity of 1300cm³. We are the more intelligent species.