Modding

Modding, or creating mods, is the act of modifying the game files in order to cause changes or create new behavior within the game. There's a lot that can be done with modding - this page seeks to be a clear outline of what modding is, what you can and cannot mod, and other important high-level concepts.

If you're looking for installing mods, check that page.

And you can check the modding tutorials page for a portal about various available, specific tutorials.

What's a Mod

A mod - short for "Modification" - can be anything user-created that edits the game in a meaningful way.

In the Total War series, mods are composed of a single file - a .pack file - which contain all of the changes within that mod. Packs act like a .zip or .rar file, a compressed folder in a single file. Much of the vanilla game is composed of various CA-made pack files.

Packs can contain a lot of various pieces, such as:

Data: One of the most-used parts of a mod is databasing. Total War uses database files, similar to spreadsheets, which can be used to change behavior and values, or add new objects to the game. This is used for much of the game, from units and their stats to the cost of buildings and much in between.
Text: On top of that, text you see in the game - from the name of a lord to the flavour text of a skill - is done within packs, in .loc files. These files attach text to the database, and they can be used for translations as well!
Assets: This is a wide topic - but much of the game can be edited through assets. Assets range from .png images scattered throughout the UI, to altered .dds files which are used on the various unit models in the game, to entirely new models or otherwise.
Battle Maps: Battle maps are created through the Terry tool, from within the Assembly Kit.
Scripts: There's a layer of the game that can be interacted with through scripts (.lua files). Scripts power things such as characters getting experience, legendary lord defeat traits, and the Chaos Invasion - and much in between!

There's natural limitations to modding the game, since the game is written in a way that cannot be decompiled easily - meaning much of the game code and engine is unreadable and untouchable. That said, Creative Assembly does a comparatively excellent job at making sure much of the game is exposed to being modified!