In Yosemite the App Store System Preferences pane has automatic updates for a few separate items:
"Install App updates"
"Install OS X updates"
"Install system data files and security updates"
App updates would include 3rd party software as well as Pages, Numbers, iMovie, etc.
I'm not clear on the other two. My guess is OS X updates includes things like 10.10.3, iTunes, Safari, Remote Desktop Client Update, Camera RAW, etc. My guess for system data files and security updates may include things like XProtect definitions, Security Update 2015-001, etc.
Are those assumptions correct?
Would 10.11.0 when it released be considered an "OS X update" and be automatically installed?
I've found these articles (one two) with conflicting info, but an haven't located an official knowledge base article from Apple with definitive answers.
Best Answer
Apple documents the difference between an update and an upgrade as follows:
Updates within each named OS include security, performance and sometimes new features. They are focused on the first two.
Upgrade is from Lion to Mountain Lion or Mavericks to Yosemite. These are primarily about large feature introductions and major changes.
Sometimes a big change will arrive as an update, but in general you have to purchase (even if it's a free redemption) upgrades whereas updates can be automated with the software update app and or the software update portion of the Mac App Store.
Historically, the two were separate, but it's a bit more confusing now since everything the normal end user sees is in the App Store app.
You can see what updates are pending by running the command
softwareupdate -lin terminal and it will only list system updates.