What is the difference between game versions?

Like minecraft goes through versions eg: 1.7.2 1.7.4 1.7.9. What is the difference? Is it the amount of code that was changed/added? Amount of time? Amount of resources?

It is just the updates, for example: 1.7.4 is newer than 1.7.2

I know this much. But what makes 1.7.4 newer than 1.7.2? What did they do? Code? Resources? What did they add or take away for it to get to a newer version? And why do they skip some numbers? Just say they have 1.7.2 and a few days/weeks/months later the next version is 1.7.4. I know they had 1.7.3 but sometimes they skip some numbers.