Service packs contained new features and changes to core files not available before or after as updates. They also contained a roll up of updates released after the last service pack. Only updates not already installed were downloaded. A service pack was a new version of Windows.

A build is a complete copy of the OS. It may or may not be a new version of Windows. So far only two Windows 10 builds have also been new versions of Windows; build 10240 (Windows 10 Gold) and build 10586 (Fall Update).

There is a significant difference between mainstream Windows and insider Windows in that mainstreamers receive cumulative updates until the next new version of Windows and insiders receive new builds between new versions of Windows. Insider builds are candidate new versions of Windows released for testing and evaluation every few weeks.

Whether it was a service pack in the old days or milestone builds now, the key is that both are classified by Microsoft as creating a different version of Windows.

Each service pack level had (or still has) its own support lifecycle. The current lifecycle table is added below.

With Windows 10 Microsoft has added a new wrinkle to the support lifecycle. In the table below you can see a significant change to the support lifecycle policy. Windows 10's lifecycle is based not on versions but on the user having applied all updates to-date. Please read the **footnote carefully. The information in the footnote may explain why Microsoft is forcing updates now.

Couple of notes:
--Microsoft did this one time before when they released Windows 8.1. Windows 8.0 and Windows 8.1 are completely different builds and are unrelated to each other, just like the Windows 10 builds.

--Until Microsoft completely revamps the upgrade process, this will be a bad way to do updates. As in any upgrade, the upgrade process sucks. Some things work after; others do not. We saw that with the 10586 upgrade in November. Some settings made it; some didn't. Some programs worked fine afterwards; others didn't. Upgrading from one build to another uses the EXACT same mechanism that is used to upgrade from Windows 7 to Windows 8, which we all know is a troublesome process.