App Designer is a rich development environment that provides layout and code views, a fully integrated version of the MATLAB® Editor, and a large set of interactive components. You can package an app directly from the App Designer toolstrip, or you can create a standalone desktop or web app (requires MATLAB Compiler™). It is the recommended environment for building most apps.
If you have existing apps that you developed in GUIDE, consider migrating them to App Designer.
Migrate GUIDE Apps to App Designer
Migrating your app allows you to take advantage of the new components and features in App Designer that GUIDE does not offer. Keep in mind that some components in GUIDE are not available in App Designer, and that you must to update your app code to make it compatible with App Designer. See the table below for details.
For assistance in migrating your apps, use the GUIDE to App Designer Migration Tool for MATLAB. For more information, see https://www.mathworks.com/app-designer/add-ons.html.
Differences Between App Designer and GUIDE
The main differences between App Designer and GUIDE are the code structure, callback syntax, and the way that you access UI components and share data.
There is a table summarizing the differences from mathworks.com.