I think a lot of the authors assume you already know what to do with the various file types. Since I haven't seen a list in the FAQ or anywhere else, here goes:
amb - ???. These seem to tell Civ3 how to syncronize a unit's sounds and animations. They go in your [Civ3]\Art\Units folders. Always back up your Civ3... uh... amb files... before replacing them!
bat - Script. Not very common, but are used to start programs with specific options. Should be accompanied by instructions for use (look for "readme.txt")
bic - Scenarios. You usually put them in the [Civ3]\Scenarios folder, but some need to replace the "civ3mod.bic" in your main Civ3 folder. Always back up your civ3mod.bic before replacing it!
bik - Movies. I don't think anyone's posting these, but they're intended to replace files in your [Civ3]\Art\Movies folder. Always back up your Civ3 movies before replacing them!
bmp - Windows grapgics. Usually just screen shots or examples, and aren't needed to use the mod/tool. Just look at them and see if it's anything interesting. Most graphics progams can read these, but not all browsers.
cpf - Civ Placement File. Used by Gramphos' Civ3 Copy Tool and Civ Placement Tool.
css - Stylesheet. Used by htm and html files.
dll - Library. Programs may come will dll files, which will need to be in the same folder as the exe in order for the program to work.
exe - Programs. They're usually tools, trainers, or what have you and will come with instructions in an included text file (especially "readme.txt"). Sometimes these are "self-extracting" files, which are like .zip files; just double-click to extract.
flc - Animations. These are usually intended to replace a file in your [Civ3]\Art folders. Always back up your Civ3 animations before replacing them!
gif - Web graphics. Similar to bmp. Most graphics programs and web browsers can read these.
htm - Web page. Usually contains documentation. More flexible than text, and can display graphics. Usually requires several files (including css, gif, htm, html, and jpg) all in the same folder in order to work. Can be opened with any web browser.
html - Web page. Same as htm.
ini - Settings. Usually used by programs the same way as dll files, but sometimes intended to replace unit settings in your [Civ3]\Art\Units folders or [Civ3]\Art\Animations folders. Always back up your Civ3 settings before replacing them!
jpg - Photographs. Similar to bmp. Most graphics programs and web browsers can read these.
jpeg - Photographs. Same as jpg.
mht - IE web archive. Similar to htm, but must be opened in Internet Explorer and it doesn't need separate files (all the graphics are in this file).
mp3 - Music. Usually intended to replace files in your [Civ3]\Sounds folders, but may also just be for you to listen to. Always back up your Civ3 music before replacing it!
ocx - Library. Similar to dll.
pcx - Civ3 graphics. Similar to flc.
rar - Archive. Contains several other files, which have been compressed to reduce their size. When extracted, the original files will be re-created. Use WinRAR to open.
txt - Text. This is the good stuff. Text files, especially ones named "readme.txt" are usually the instructions for what you just downloaded. When you recieve a text file named "readme.txt" or some other common name, it's best to rename them to something more recognizable. This also prevents future downloads from overwriting them! Certain text files also may be intended to replace files in your [Civ3]\Text folder. Always back up your Civ3 text files before replacing them!
wav - Sound. Similar to mp3, but may also be placed in your [Civ3]\Art\Animations folders. Occasionally required by programs the same way as dll files.
zip - Archive. Similar to rar. Use WinZip or WinRAR to open.
Jan 24: bolded the extensions to make them easier to find
Jan 5: added 'amb' type
Jan 3: added more web types
If anyone has any suggestions/corrections/additions, let me know!
