Mods are installed in a special folder separate from the official game, and you need to activate the mods once you see the main game menu. You can at any time return to main menu, then start the original game without mods.
Civ 5 Main Menu > SINGLE PLAYER / MULTI PLAYER > Original Game
Civ 5 Main Menu > MODS > SINGLE PLAYER > Play with Mods
So you choose whether you want to load mods each time you start Civ 5.
As Ozzy above noted, some mods will affect savegames, so some savegames may require that you activate a specific version of a specific mod in order to continue playing that specific save.
A second note: some MODS that require you to make changes to the main game in the Steam\steamapps\common\...etc etc...\ may be permanent and unless you backed up the original game files, would require a reinstall to go back. As long as you never run "installer" type of MODS outside of the main game menu, and never make changes to anything under the Steam folder, you're all safe. Mods are supposed to go into your C:\Users and Documents\<your user name>\My Games\Civ 5\MODS\etc.etc., so they never make any changes to the original game files. Any mod that you install via the in-game MOD BROWSER would be safe.