What are PDM, APD, Divergences and VRPP?
PDM - The Pop Demand Mod is one of Victoria II's biggest, most successful mods which is always ongoing and even has its own dedicated subforum (found
here). PDM originally focused mainly on rebuilding and rebalancing the Victoria II economy, but has it has evolved many other changes have been made to systems ranging from national unification to combat. You can find out the details of what it changes
here.
APD - APD is PDM (Pop Demand Mod) for the A House Divided expansion pack of Victoria II.
Divergences - Divergences is Victoria II's largest, most famous perhaps its only completed AlterVicky mod. Divergences alters the map to an entirely different timeline, while also rebalancing power blocs in order to avoid such static places as India in Victoria II vanilla. It is a world encompassing mod, worth trying out. All the details of the history behind it, and details on where you can download it can be found
here.
VRRP - The Victoria 2 Realism and Rebalance Project is a vast mechanics mod, fine tuning Victoria II's economy, system of war and various other features of Victoria II. It's dedicated subforum can be found
here.