The ninth major release of Umbraco CMS contains some very big changes. Firstly, the underlying framework has changed from ASP.NET MVC/.Net Framework to .NET Core/.NET 5. Not only does this allow Umbraco 9 to leverage the multiple performance improvements of .NET Core but it also gives cross-platform compatibility meaning Umbraco projects can be developed much easier on non-Windows devices such as MacBooks.
As part of the move to .NET Core there have also been changes to some of the packages and dependencies Umbraco uses. For example, ImageProcessor is now ImageSharp, Client Dependency Framework (CDF) is now Smidge and Examine has been updated to use a newer version of Lucene.NET that is compatible with .NET 5 and Umbraco 9.
More information on the changes in Umbraco 9 can be found on the Umbraco website.