Linux Mint 18 will be a long term support release, which means it’ll be officially supported through 2021, and the new version has a new theme called Mint-Y, updated software, packages, and a new set of default apps called X-Apps.
X-Apps are basically apps designed to run on desktop environments that use GTK, allowing the same apps to run on MATE, Cinammon, and other desktop environments that grew out of GNOME.
Some of the first are basically modified versions of existing applications. For example, Xed is a text editor based on Pluma. Xviewer is an image viewed based on Eye of GNOME. And Xplayer is a media player based on Totem.
Other changes in Linux Mint 18 include update managers that allow you to install kernel updates the same way you would install software updates, a new Mint-Y theme with a simpler, flatter look than the Mint-X theme (which is still available).
Linux Mint 18 also has better support for high-resolution displays and the Software Manager now includes additional popular apps including Steam, Spotify, and Minecraft.
Keep in mind, this is still a beta release. The stable version of Linux Mint 18 is expected to come next month.