Huge patches are all the rage of late.
The change-list is absolutely gigantic, and I don’t think it would serve any useful purpose to recreate it in full here. Head to 2K’s own forums if you want to read the patch novella.
In summary, it takes Civilization V to version 18.104.22.168 and brings improvements to the AI, gameplay, diplomacy, the user-interface and modding.
The update should auto-download for you when you fire up Steam.