A live demonstration of getrandombeacon: pick a range and how many
unique numbers you want, and the result is decided by real GPU mining work, not a server-side
random number.
Starting a draw locks in the current block height. If you pick a draw time, nothing about the eventual result is decided in advance, this server just waits in real time until that moment, then looks at whatever the actual chain height is right then and uses a few blocks after that. The numbers are decided by mixing together the mix_hash values of blocks that don't exist yet at trigger time, so whoever runs this page cannot know the result in advance, however far ahead the draw is scheduled. The miner who finds the last of those blocks sees it first and could discard it; here is how much that can matter.
Want to use this in your own code? See the JSON API.