I mean for example if a software has this number in its version 13.0.5 and another version has 13.0.5.1 or something like that what is that zero for?what does this zero show? Is there a rule for these numbers?
Yes it appear because there is a revision number after it. If it's been 13.0.0 then that'll be 13.0 . Major and Minor revision must always show, but the revision and build number are not mandatory. That said, there's many ways to manage version numbers. This should help you figure out : http://en.wikipedia.org/wiki/Software_versioning Hope that help. 


The Wikipedia reference from Marc on Software Versioning is good, 


What do the numbers in a version typically represent (i.e. v1.9.0.1)  Stack Overflow How to do version numbers  Stack Overflow Version numbering basics  Stack Overflow 


Or modern style Version 1 

