I've seen a gazillion posts regarding this but have no idea what it actually does. Is it a whole different recipe that you add to the cake mix/recipe?

Nope it's nothing you need to add to the cake it's for sheet cake pans and it makes cakes rise higher without having to use more batter. I've used them alot.
HTH

It is actually for any recipe that you want to get more batter from. Good to use with 3" pans or if you want some extra cupcakes on the side, etc.

I have usually seen recipes for use with cake mixes.

An extender is different than a "doctored" mix. With an extender, you actually get MORE batter by adding additional flour, sugar, fats, etc. to the traditional recipe.

With a "doctored" mix, you still get pretty much the same amount of batter but you add some custom ingredients to the standard box directions to make it taste better, be more moist, etc. This is usually my method of baking cakes - I use mixes, use milk instead of water in the amount called for on the box, same oil/butter as called for on box, use an extra egg (meaning usually 4 instead of only 3) and I also add a box of instant pudding powder.

That would come in handy since I need to make a 1/2 sheet in a few weeks.