BUG (BTS unaltered gameplay) mod is an interface mod that brings a lot of the more obscure information in the game mechanics out out front, and redesigns elements such as the scoreboard and most of the advisor screens (including major overhauls to the Espionage and Domestic advisor screens). IMO it really makes the game a lot more palatable as I don't like the default appearances and am a bit of a complex-o-phile. A common example: BUG has handy features like the "red fist" on the scoreboard which make it easier to immediately see when an AI is gearing for war; but it doesn't tell you the target, and you can only see it when you could normally check for "We have enough on our hands right now" in the diplomacy screen. Hence, unaltered gameplay.
BULL is a replacement .dll that from what I understand behave like BUG but has even more features that can potentially (?) change gameplay decisions, like actual calculated yields on improvements/buildings, hidden attitude parameters on the leaders, and more in-depth combat odds information. BUG mod will let you know exactly which features BULL can add in its own options menu, such as: stopping chops on the turn they are due instead of automatically completing them, turning off some of the other automatic actions of the game (such as asking for war declarations every time you move onto a rival unit, certain alerts, etc) and a mapfinder tool.
Never used BAT but according to its page it introduces some graphical overhauls to models and I think it's the one the adds more buttons in the city management screen. No experience with this one, sorry.
There was a thread/article somewhere discussing acronyms, I'm looking for it.
EDIT: here
https://forums.civfanatics.com/threads/civ4-lingo.631269/#post-15097990
But for now some really common ones:
SiP: "Settle in place;" as opposed to moving from the starting location
WHEOOHRN: "we have enough on our hands right now;" the message the AI gives you in the diplomatic screen when you mouse over a potential target to bribe them into war with. It means the AI is actively plotting war or already at war with somebody. They will not accept bribes into war in this mode and is usually an indicator of danger if you are an acceptable target and aren't already at war!
WFYABTA: "We fear you are becoming too advanced;" the AI tells you this message when they won't trade techs with you because of their personality specific (each AI has a different thresehold) limit of watching your total number of trades has been reached. It mostly factors on higher difficulties where it is reached more quickly. You own vassals always trade no matter what, Friendly status with an AI ignores it too (though they may have other reason not to trade) and Mansa Musa always trades unless you're his worst enemy.
Tiles are also often referred to using cardinal directions in relation to one another (i.e a tile 1S2E) and in terms of their yields, hammers(h), food(f) and commerce(c).
I'd also recommend keeping this amazing article handy for reference:
https://forums.civfanatics.com/threads/civ-illustrated-1-know-your-enemy.478563/
