Netrom, you are in fact in the wrong place, this is a very old sticky about python modding (predating Civ IV) but to answer your question, do you have BtS with the latest patch, 3.17? If your mods are for another version they may crash. I got a new computer, installed a big 1 gigabyte mod and spent a month making a mod of my own, then
1. the mod I downloaded (New Destiny) crashed
2. I uploaded my mod and realized I had not patched my new computer to 3.17 and had made a mod for a version that was obsolete.
Leoncalvin definitely XML. Start out like this. Make a folder called MyMod. Inside it make another folder called Assets and inside that make another one called XML. Now explore into the program files/Civilization/BeyondTheSword/Mods. Put your folder there. Now, go out to ProgramFiles/Civ/BtS/Assets/XML. You should see Art, Units, GameInfo, Terrain stuff like that. Whatever you want to change, copy it and paste it into an identical directory structure in your own mod, the folder you made in step one. Say you want to increase the combat strength of the basic tank from 28 to 30.
Make a folder called Units in .../MyMod/Assets/XML. Into ProgramFiles/CivilizationIV/BeyondTheSword/Mods/MyMod/Assets/XML/Units copy the file UnitInfos.xml from ProgramFiles/CivilizationIV/BeyondTheSword/Assets/XML/Units. Now, open your UnitInfos with WordPad. Now use the search, looking for UNIT_TANK until you get to the actual part that talks about the tank rather than something indented about things upgrading to tank or something.
Now search, looking for iCombat. This takes you to a "tag," which is a line in XML.
<iCombat>28</iCombat>. The / in front of the second iteration of "iCombat" means the tag is ending. (A slash at the other end, without both bookends, means a tag about a null value.) What matters here is what is in between the book ends, namely the number "28." By changing this to 30 you make the basic Tank more powerful. Save this changed document. Now start Civ and Single Player/Play Now go to Advanced/Load Mod. Select "MyMod." Now you are playing a little mod with Shermans retrofitted with the 105. I think they called it the Pershing.
That's basically how it works. Learning to mod XML is basicallly learning what stuff is and where it is.