A gaiwan or gai wan ("lidded bowl") is a Chinese covered bowl used for the infusion and drinking tea. It is also known as gaibei ("lidded cup") or juzhong ("hot-steeping vessel").
Tea experts think that the gaiwan is a great way to brew teas with delicate flavours and aromas, such as green tea and white tea. Gaiwan is also used for preparation of oolong infusions beacuase they can be infused several time. Still, you can use gaiwan for any type of tea.
The gaiwan is also important in tea tasting because of its open and glazed surfaces.
The gaiwan consists of a saucer, bowl, and lid. The gaiwan itself can be made from myriad materials, from porcelain to glass. Gaiwans made from Yixing clay or jade are especially valued by collectors.

In the folowing video you can see preparation of oolong tea with gaiwan.