Hugelkultur is a German word meaning hill culture or hill mound. It is the ultimate raised bed.
It means you can grow a typical garden without irrigation or fertilisation. It is quite simply buried wood, that you cover in manure and then compost and then mulch. It means it uses up rotting wood, twigs and branches. You can use whole trees but that would be a big bed. It can be small and it can be big (whole trees) and in our back garden we have started small and will be getting bigger.
The woody matter absorbs water and keeps everything irrigated and the rotting wood also provides plenty of goodness to the soil. Over the years it just gets better. And if like ours you keep building on and on it will just keep getting better.