Arma: Mod Folders
Custom modfolders are folders that can contain theme addons for easy selection, or Data files which change general parameters of the game. If you have OFP:Resistance, your \RES folder is an example of a modfolder.
Some Flashpoint players prefer to put an "@" in front of each modfolder's name, so they then get listed at the beginning of the main Flashpoint folder, and can be easily separated from essential game folders.
To use Modfolders you have to create a shortcut to your OFP executable, and in the path add the following parameter: -Modname
"C:\Program Files\Codemasters\OperationFlashpoint\FLASHPOINTRESISTANCE.EXE" -nomap -nosplash -mod=@winter
Alternatively, you can use 'OFP Mod Selector', by Operator of Dissidence.
Or you can get Kegety's launcher here. 
Within the Missions folder you can use subfolders, like in the example shown here. This cannot be done in the MPMissions folder. You don't have to use a Mod to make such subfolders, you can create them also just to sort your missions.
Within the MissionModFolder, you can place an overview.html which, just like Single missions, can display an image and some lines of text.