What's the difference between the POI Modes and the DMA modes in the BIOS?

I have a VH6T, and want to know as much about the BIOS as possible, especially the part dealing with the IDE controllers.
You can set the different devices to POI modes 1 to 4 and also their DMA mode.
what do these different options do?

PIO & DMA Modes for IDE devices refer to how the devices communicate with the IDE controller. PIO is older/slower & DMA is newer/faster. PIO modes are PIO 1 thru 4 (4=fastest).

You can put 2 devices on each IDE channel & for best performance both devices should be rated the same. If you put an ATA-100 hard drive on the same channel as a PIO CD, both devices will operate at the PIO speed.