Donations are made using real-world money and help maintain the network. In exchange, donators gain reward points as thanks which can be used in the reward store to purchase various items of convenience.
All pricing is given in USD as a standard, but many currencies are accepted including EUR, GBP, and AUD. It's generally advisable to buy more points in one go as higher values are more economical, in fact the rate per dollar climbs from 20 to 60 towards the bottom of the table. These prices are potneitally subject to change.