Unless you keep your programs only for yourself, you would want to give them a version. Versioning is a popular technique where a unique version number is assigned to a program to denote a specific state of the code of that program at a specific time. A version number is used as a reference to what the program, as a deliverable, includes in terms of functionality. This reference is expressed in words that your users would understand. Furthermore, if your users experience a problem, you could try to reproduce it on that same state of the code.
There are many versioning schemes that dictate how to generate new version numbers. When trying to implement continuous integration (and delivery), though, you may face some challenges. In this blog post I will try to explain some of these challenges and what options there are.