A rollup is just a combination of individual patches into a single update, and will help reduce micro-management. It will apply to all versions of the Framework and will coincide with Windows.
However, updates will only apply to your current install, and won't upgrade the base version to higher build number. A separate security-only patch will also be available each month, although it's only accessible via ESUS and SCCM.
Reasons for the Change
Microsoft don't go into specifics with the reasoning for .NET Framework, but we can assume that it's due to the same core problem: update fragmentation.
Due to the previous freedom to install whatever updates you like, many PCs are running software with different updates applied. This makes it harder for both admins and Microsoft to maintain a stable environment.
Switching to a monthly update system for .NET Framework should help to reduce this problem, whilst also simplifying the whole process. The downloads are expected to be around 60MB.
The company states that the previous method also led to many other problems:
“Various combinations caused sync and dependency errors and lower update quality
Testing complexity increased for enterprises
Scan times increased
Finding and applying the right patches became challenging
Customers encountered issues where a patch was already released, but because it was in limited distribution it was hard to find and apply proactively.”
Changes are also coming to the Microsoft Update Catalog to make it even easier to manage and find the right installs. The primary goal is to remove the Active X requirement so that its more accessible to different browsers and systems.
The .NET Montly rollup will be available for Windows Vista SP2, Windows 7 SP1, Windows 8.1, Windows Server 2008 R2, Windows Server 2012 and Windows Server 2012 R2. You can read the full list of changes on the blog.