https://www.cnet.com/news/galaxy-s10-vs-galaxy-fold-whats-the-difference/

The biggest difference between the two Galaxy phones is the design. While Samsung tweaked the Galaxy S10's design a bit compared to the S9 (more on that later), the Galaxy Fold represents a new form factor altogether.

The Fold, if it's not obvious already by its name, has a foldable OLED screen. When closed it has a secondary 4.6-inch display that serves as its "cover," with all the usual features you'd expect on a phone. When you're ready for something bigger, the Fold opens up hot-dog-style like a book to a 7.3-inch tablet.