The purpose of manufacturing yellow tea is to remove the grassy taste of green tea, and to make the leaves take on a yellow coloration.
The grassy flavor is an intrinsic quality of green tea, and is a result of the tea leaves being minimally processed after the harvest. In essence, the leaves retain most of the substances present in the leaves when they are still on the bush. The chlorophyll in the leaves both gives the rich, green color, and provides the characteristic flavor.
During the process that leads to the tea leaves becoming yellow tea, the chlorophyll is broken down and partly oxidized. The taste of the tea becomes milder, and is often described as being somewhere between white tea and green tea. The aroma is described as flowery, fresh and mild.
Yellow tea is typically harvested early in the year, before most green teas are harvested. This means that the leaves are younger, that they are still rolled up into buds, and that the leaves are smaller when rolled out. While green tea is often cut and ground, yellow tea is always sold in whole leaves, often in thin buds.