A service pack (in short SP) is a collection of updates, fixes and/or enhancements to a software program delivered in the form of a single installable package.Many companies, such as Microsoft or Autodesk, typically release a service pack when the number of individual patches to a given program reaches a certain (arbitrary) limit.
Updating is completely automatic via Windows Update.
You can read more about the individual fixes and improvements on Microsoft's Windows 10 Update History: Version 1703 page.
As of August 2017, the latest major update for Windows 8 is Windows 8.1 Update.
Much like updates on your smartphone, Microsoft is continually adding major features via automatic patching.
Below you'll find all the latest information on both service packs and these other major updates that Microsoft is regularly pushing to its users.