During centrifugation, the emulsion for DNA extraction separates into two distinct phases. Chloroform:octanol is more dense than water solutions, so it forms the lower (green) layer. It is also more chemically attractive to molecules such as proteins and polysaccharides. These are thus separated out from the DNA, which is contained in the upper aqueous phase. This clear solution is carefully transferred to fresh centrifuge tubes using a pipette.
Photo credit: CIMMYT.
See the "DNA extraction" set that this photo is part of for more information and images.