A siphon is a tube that has one end below the other. Fluid passes through the tube to the lower end by gravity. Consequently, an underpressure is created at the upper end of the tube causing more fluid to flow into the tube. As more fluid passes through the tube again, the process becomes continuous.

The idea behind a siphon is to transport a fluid from a higher level (1) to a lower level (2) while passing a 'treshold'. The treshold is at a higher position than #1. As gravity + pressure difference have to overcome the treshold potential energy, point 2 has to be sufficiently lower than point 1. Please mark that it makes no sense to compare the level of point 2 with the height of the treshold.

That's a good answer, but doesn't mention air pressure. It's the air pressure that pushes the liquid upwards into the tube from the higher vessel.

Would a siphon work in a vacuum? Most liquids boil in a vacuum, so that's probably untestable. Does anyone know the answer?