The Creation Engine is a game engine developed by Bethesda Softworks for personal computers and NextGen consoles. The game engine was used in The Elder Scrolls V: Skyrim, Fallout 4, and Fallout 76, with progressive improvements to the engine being made between each game's release.
The Creation Engine is a collection of in-house tech and middleware tools, developed based on the Gamebryo iteration used for Fallout 3 and Fallout: New Vegas. For Skyrim Bethesda honed or replaced every major system powering the gameplay experience, however, retaining compatibility using exactly the same file architecture of the original Gamebryo and NetImmerse engines, as well as the cell-based gameworld (interiors are loaded separately from the outdoors). Fallout 4 uses a further modified Creation Engine.
- Features since Skyrim
- Improved physics, a more powerful and effective renderer, scalable level of detail for distant landscapes, particle effect and other improvements. 
- New foliage system. Allowing to render realistic trees and foliage, increasing verity and allowing for weather effects such as strong violent winds or blizzards and their effects on the environment displayed. Its an in-house tech that replaced SpeedTree.
- New precipitation system analyses the geography of an area with the goal of getting snow to accumulate properly on your surrounding environment.
- Havok Behaviour technology is used for character animation. It allows for more nuance in character and creature movement, govern special effects etc
- Radiant AI system, has been updated with more complex subroutines, improving NPC decision-making abilities and daily routines. 
- Radiant Story, an extension of the Radiant AI system, a dynamic quest system which can alters side quests structure depending on your actions, in an attempt to make random encounters feel less forced or arbitrary
- Features since Fallout 4
- Supporting Physically-based Rendering (PBR) and a dynamic volumetric lighting system
- character editor system that allows a free-form creation of faces.
The Creation Engine comes with a suite of development tools for modders and integrates with the Steam Workshop to allow users to share, rate, and download mods.
The Creation Kit is the name of a free to download suite of development tools used at Bethesda Game Studios to create Skyrim and Fallout 4.
The Creation Kit is a program that can be used to control nearly any aspect of the game or create mods, capable of adding brand new user-created game content, examining the game data and retexturing the game. With the help of third-party engines, it can also change GUI elements such as making a PC-friendly items menu.
Prior to the Creation Kit release, many modders were using third-party tools to indirectly extract and then read the game's files. Additionally Bethesda has rolled out a new Wiki for modders, providing them with tutorials and information about the tool.
The Steam Workshop integration allows for Mods to be directly uploaded from the Creation Kit to the Steam Workshop. This allows players to quickly install new mods. Players can directly subscribe to the mods from the Steam Workshop. Every time the game launcher is started, it will automatically download the latest version of any subscribed mods.
- Creation Kit Wiki -- Official wiki for the Creation Kit where you can find details and tutorials for creating mods.