A corkage fee is the price you pay to
DCL for them to pull the cork on your bottle of wine. Basically it's an equalilzer - while DCL will let you bring your own drinks on board, they also make money by selling you drinks - so this is how they make their money - basically they penalize you for bringing your own.
Corkage fees are not uncommon in eating establishments that also let you BYOB (where laws allow), but as these are not an option in the US, many cruisers people haven't encountered the term. I know Australia, being an enlighted country as regards things like this, also allows BYOB, but I don't know about any other countries.
There are 2 different wine packages available - I'll have to look thru our stuff to see if we still have any info. There's the standard package and the premium package - but I'll not bother to quote prices, since I don't have the info on hand.
The wine list can and does change; as I said, I thought the latest list was better as it offered more varieties from vinyards in the US that I was at least familiar with. There's nothing wrong with a good French or German wine, it's just that I don't know enough about the wines from those countries to always make a good decision for the meal (although, this is an area where your assitant server can frequently help out).
As far as quantity goes, the packages basically let you purchase one bottle per day for each day of your cruise. If you purchase the standard package, you can upgrade individual bottles to the premium package - kindof a 2 standard for 1 premium (I don't know how they do cash upgrades, we just happened to end up with 2 bottles on the last night and upgraded to a single premium based on the recommendation of our assistant server, and were very pleased with the suggestion). If you don't finish a bottle one night, the assistant server will save it for you until your next night.