While Catalina includes several new features, it is primarily a performance update. Notably, Catalina is the first version of macOS to drop 32-bit support. By removing support for 32-bit applications, Apple trimmed a significant amount of "legacy" code from the OS. The result is a lighter-weight operating system optimized for 64-bit applications.
Catalina is also the first version of macOS that allows an iPad to serve as a secondary display. This feature, called "Sidecar," works with both a wired and wireless connection between a Mac and an iPad. It also introduces "Project Catalyst," which makes it possible for iOS and iPadOS apps to run on macOS.
Most of Catalina's other new features are related to the bundled apps. For example, iTunes is replaced by three new apps: Music, Podcast, and TV. Mail adds message blocking and built-in unsubscribing. The Reminders app provides a new way to organize and manage reminders. Catalina also includes Screen Time, a feature introduced in iOS 12. It allows users to track how much time they spend on their computers, including how much they use individual apps.