Comparing Mouse and Hamster, mouse has a long tail (almost as
long as its body length) whereas the hamster has almost no tail...A
hamster looks more like a fur ball too... 
Rat is much larger than mouse and hamster, serveral times larger
when full grown.
If you put 2 (stranger) male mouse into the same cage, they will
fight. But 2 (stranger) male rat in same cage will not fight.
Female rats will fight i think.
