You get them from DLP. You can order them at the time of booking or you can phone up and have them added on any time before your visit, but I don't think you can get them once you arrive.
There are 3 options, halfboard standard, halfboard plus or halfboard premium and they all have a different face value. I don't know what the values of the others are, but I asked earlier and was told that adults are worth €25 each and childrens are worth €11 each.
You have to purchase them for everyone on the booking (except children under 3), and you have to have them for the complete length of stay (number of nights, not days). You can only get them if you're staying in a Disney Hotel though, although I don't know about partner hotels I'm afraid.
Each option has different restaurants that you can use them in, but if you wanted to use them elsewhere you can use them at face value and just pay the difference. They can't be used at non-Disney places though - Planet Hollywood, Rainforest Cafe, King Ludwigs.
With the exchange rate being what it is for the GBP it was definitely worthwhile for us. We paid £260 for 4 of us for 5 nights and the value of the vouchers we will get is €360 and at least we know that if all else fails, we'll have at least one good meal a day lol.
Hope this helps