It is exactly like trying to find out the difference between a small piece of sugar candy and a large one. Can there be any difference between the two?
The living entity that has got separated from the Supreme Lord (Paramātma) is called atma. All living entities have to ultimately unite again with the Paramātma. This can be compared to a bead string. Paramātma in the form of the string is holding all the atmas (beads) together. However each bead is different; it has its own identity.
Based on the karmas of the past births, each being acquires different qualities, traits, physical looks etc. Even though one being may differ completely from another, both in looks as well as traits, the God that exists in both is the same! However the living entity considers itself as separate.