.NET Core and .NET Standard are both part of the .NET ecosystem, but they serve different purposes and are used in different contexts. Here’s a breakdown of the differences between a .NET Core Class Library and a .NET Standard Class Library:

.NET Core Class Library Target Framework: Specifically designed to run on the .NET Core runtime. Features: Can leverage APIs and features that are unique to .NET Core, including some that may not be available in .NET Framework or .NET Standard. Use Case: Best suited for applications that will only run on .NET Core or that require specific .NET Core features. Performance: Generally offers better performance optimizations specific to .NET Core. .NET Standard Class Library Target Framework: An API specification that defines a set of APIs that all .NET implementations (like .NET Core, .NET Framework, Xamarin, etc.) must support. Features: Aimed at creating libraries that can be used across multiple .NET implementations. It doesn't provide platform-specific features. Use Case: Ideal for creating reusable libraries that need to work across different .NET runtimes. Compatibility: Ensures broader compatibility but may not leverage the latest features in specific platforms like .NET Core. Summary Use a .NET Core Class Library if you are targeting only .NET Core and want to utilize its specific features. Use a .NET Standard Class Library if you need compatibility across multiple .NET implementations and want to ensure your library can be reused widely. As a general practice, if you are starting a new library today, you might consider targeting .NET 5 (or later), which unifies .NET Core and .NET Framework into a single platform, providing a modern development experience.