Factorio supports changing the game properties and logic via modding in Lua. The community has created quite a lot of interesting stuff already.
The official place for hosting, browsing and downloading mods is the Factorio mod portal.
Once you have an account, you can access the mod portal from inside the game, where installation is as easy as clicking 'download' and waiting for it to finish. Once you have finished all the downloads, the game will restart and enable the mods.
Again the best resource of information is the forum and wiki. Check out the general introduction to modding at our wiki.
There is also an up to date reference documentation of the modding api. This is generated directly from the Factorio codebase, and is updated for each release.