The simple answer as to the difference is 2018-2015 = 3.
If you want a better answer, then you need to do some reading, as there are a HUGE number of differences.
Depending on what toolboxes you own, many of those differences may be completely irrelevant.
So look at that page, then look at the changes from R2015 to each suceeding release.
To a large extent, in terms of the language itself, the differences are not huge. Code written in an old relase will very likely still run. But only you know what is important to you. Only you know which toolboxes you have.
