One of the key differences between Windows 10 and previous versions of Microsoft's operating system will be the way the it updates itself. Current versions of Windows receive security updates on a monthly basis and periodic non-security related updates. With Windows 10, Microsoft will put the old fiction of not making feature changes to shipping operating systems to bed.
Windows 10 will have a monthly update cycle for security fixes but it will also have a a steady stream of feature updates. For feature updates, there will be three different release cycles. Consumer installations of Windows 10 will receive new features as soon as they're available. Regular businesses can choose an option that lets them control when those new features are rolled out so they don't disrupt business processes and admins can even opt out of feature updates completely for mission-critical systems and environments.
Our understanding is that even with this continuous delivery of new features, Microsoft will occasionally update the Windows version number. This will both provide a name to attach to big updates and an update route for those using the most conservative update policy.
In conjunction with these changes, we're likely to see the end of "Service Packs" and their corresponding installer. Individual hotfixes and the full operating system installer used for bare systems will cover all the install and update scenarios.