What is the difference between a integer that is a string and a number that is just a string?

What is the difference between a integer that is a string and a number
that is just a string?

numbertwo = int(str(numberone[2]+numberone[1]+numberone[0]))
the above works but the bottom when run gives me an error why what is the
differnce
numbertwo = str(numberone[2]+numberone[1]+numberone[0]))