.NET Core is a set of runtime, library and compiler components which can be used in various configurations for device and cloud workloads. Cross-platform and open source, .NET Core provides a light weight development model and the flexibility to work a variety of development tools OS platforms. .NET Core is available on GitHub under the MIT license. .NET Core refers to several technologies including .NET Core, ASP.NET Core and Entity Framework Core.
.NET Core has major and minor releases which represent related but separate release streams. Major releases will be highly stable, low change releases. A major release will be supported for three years after it ships, or 12 months after the next major release ships, whichever is shorter. For example, .NET Core 2.0 is a major release and .NET Core 1.0 continues to be supported for 12 months after the release of .NET Core 2.0.
A major release may have multiple minor releases. For example, .NET Core 1.0 was updated with .NET Core 1.1. Similarly .NET Core 2.0 will be updated with .NET Core 2.1. Minor releases represent a faster rate of change and innovation and will be supported within the same three-year window as the parent (major) release. Customers must be at the latest minor release level within three months to continue to be supported. For example, once .NET Core 1.2 is released customers must update from .NET Core 1.0 to .NET Core 1.1 within 3 months.
In some cases, installing a third-party component or library through NuGet may be a pre-requisite for using .NET Core. Support for these components will come from the vendor that ships those components or libraries.
.NET Core releases for certain platforms such as Red Hat Enterprise Linux will be published by Red Hat instead of Microsoft. For these platforms, updates for .NET Core will be made available to the vendor for deployment through their systems.
.NET Core and the .NET Framework (generally) have a subset-superset relationship. .NET Core is named as "Core" because it contains the core features from the .NET Framework for both the runtime and framework libraries. For example, .NET Core and the .NET Framework share the GC, the JIT and types such as String and List<T>.
.NET Core was created so that .NET could be open source, cross platform and be used in more resource-constrained environments.
.NET Core is a newer development platform and releases for .NET Core follow an agile, more rapid release cadence. Applications based on the .NET Framework are widely deployed across hundreds of millions of computers with a high bar for compatibility and stability. This results in less frequent releases for the .NET Framework with fewer changes. Due to these differences, .NET Core releases follow their own lifecycle.
If you are using the .NET Framework 4.5.x or .NET 4.6.x, you do not have to update your application to work with .NET Core. That said, the .NET Core development platform offers several advantages over the Microsoft .NET Framework. You can find more information here.