A virtual number is a telephone number that is not directly associated with a telephone line, and is usually redirected to another number. This type of number has the added advantage of being easily and quickly redirected to another number via a web interface.
There are several reasons why a virtual number can be used. Among them, most notably, is not disclosing a cell number publicly, or redirecting calls to other destinations according to a given schedule, or making sure to always keep your number regardless of your phone service provider! In any case, your virtual number can follow you everywhere!