DMA - Descriptor operation wtih example

Please any body detail describe the DMA - descriptor principle of working etc.,

ya please... me too want to know what is a DMA and its functions...
especially on the FreeScale ColdFire controllers V2... In MCF52235 datasht, it s specified as " 4 channel DMA ". what does it mean?
hav 2 start from the scrtch regardng ths issue... any help wil be apprciatd..