This update comes simultaneously released on both editions of Minecraft, as "1.18 Experimental Snapshot 1" on Java (which must be downloaded from the Minecraft website instead of the launcher) and as part of version 1.17.10 on Bedrock (available under a toggle).
The changes available in these versions include complete overhauls to cave, mountain, and terrain generation. Caves are now massive and mountains are much taller and now come in more varieties.
Specifically, there is now an increased height limit, five new mountain biomes, three new types of cave, and new ore distribution.
Mojang clarify the experimental nature of these preview versions by saying
everything is subject to change [and] some features may be significantly changed or even removed in the future.
These are very early previews of the 1.18 update – Mojang says proper development versions for Caves & Cliffs Part II
will probably start sometime around September, so there is still a two-month wait for new features.
Version 1.18 is set to release at the end of this year; Part I of Caves & Cliffs was released in June.
For a visual overview in the changes in the Java Edition version, see my video below: