Each new version of the .NET Framework retains features from the previous versions and adds new features. The CLR is identified by its own version number. The .NET Framework version number is incremented at each release, although the CLR version is not always incremented. For example, the .NET Framework 4, 4.5, and 4.5.1 include CLR 4, but the .NET Framework 2.0, 3.0, and 3.5 include CLR 2.0.
In general, you should not uninstall any versions of the .NET Framework that are installed on your computer, because an application you use may depend on a specific version and may break if that version is removed. You can load multiple versions of the .NET Framework on a single computer at the same time. This means that you can install the .NET Framework without having uninstall previous versions.
The .NET Framework changes over time. Each new version adds new types and type members that provide new functionality. Existing types and their members also change over time. For example, some types become less important as the technology they support is replaced by a new technology, and some methods are superseded by newer methods that are either more convenient or more full-featured.
The .NET Framework and the common language runtime strive to support backward compatibility (allowing applications that were developed with one version of the .NET Framework to run on the next version of the .NET Framework). This makes it difficult to simply remove a type or a type member. Instead, the .NET Framework indicates that a type or a type member should no longer be used by marking it as obsolete or deprecated. Deprecating a type or a member involves marking it so that developers are aware it will go away and have time to respond to its removal. However, existing code that uses the type or member continues to run in the new version of the .NET Framework.