When you upgrade you mac from Mavericks to Yosemite you’ll notice all the new styling changes that have come with it. One thing that can be a little confusing is that the behaviour of the green button in applications has changed. In previous versions it was used to enlarge the application window but in Yosemite it will cause the application to enter full screen mode.
If fullscreen mode is not what you are looking for then you need to use alt + green button to get the same behaviour as before.