What does alien number mean?
In the US, an alien number refers to the alien registration number. It is an 8 or 9-digit number.
There is not much difference and there is a LOT of difference! A visa number is assigned to a person entering the USA. It later can become his/her Resident Alien number if alien status is granted. An alien number (usually from your visa number) is assigned to you when you have been granted legal Resident Alien status.