"Static" and "Living" are the two options you can choose from when starting or modifying a world in Creative Mode.
In static mode, falling blocks are not affected by gravity. So you can build a structure from gravel or sand. Plants do not grow unless you plant them yourself and animals do not spawn without hatching a spawner egg. Some rare animals do not have a spawner egg even in creative mode. And Lightnings doesn't cause explosions and doesn't set on fire blocks, however, they can still burn Entities.
Changing Static/Living Environments
Note: Static/Living environments can only be changed if the world is in Creative Gamemode. If the world is on any other mode it will automatically spawn a living environment that cannot be changed.
Select single player on the main screen of the game. Select the world you wish to change the environments of (It should be highlighted green) then press the button on the bottom right corner of the screen; it should look like three dots in a horizontal line (Circled in red).
Change the environments to "Static" or "Living" by using the button circled in red.
Go back to the "Options" page then press "Apply Changes".
1.5- Birds were added, created living mode.
1.12- Flat worlds were added. They don't have plants and are completely flat.
1.16- Static mode was added. Sand and gravel now fall unless in the new mode.
1.17- Fixed animals spawning on static mode.