I know that some "addons", such as Kegety's Hi res sky addon or the one that replaces the ground textures on CWC islands with Hi-res ones, use dta/HWTL or one of those folders. Basically, if I understand it correctly, you can set up a mod folder with those folders inside of it (dta/bin/hwtl), and when you load that mod, OFP will load from those folders, instead of OFP's default folders.
If you un-pbo some of the files in those directories, you'll see that they hold pretty much all the "visible" aspects of the game (everything but the coding, pretty much). It has the animations, the land/sky textures (hence the above addons), the models, the sounds, etc. The bin folder has the game's config.cpp (config.bin), which contains many settings such as how tough units are, settings for weapons, the sound a unit makes when it is hit, etc. Think of it as the data for the "default" BIS addons, but plus more stuff. If you've seen the FDF mod or the ECP mod, you'll notice that things can be changed such as where buttons are on the main screen or the editor, etc. This is also configured in this file.
I'm sure there is other stuff in there too, but that's about all I know about off the top of my head.