A small, perforated, basket-like container with a hinged opening. Loose tea is placed inside the infuser, which is then closed and lowered into a teapot, whereupon boiling water is added. The tiny holes in the infuser allow the water to interact with the tea leaves.
A tiny chain with a hook at one end is attached to the top of the infuser, the hook slips over the rim of the teapot so the infuser can easily be retrieved, thereby straining the tea leaves.
There are also single-cup infusers, which are shaped like two perforated teaspoons that fasten together. Tea infusers are usually made of stainless or chromed steel, although there are also porcelain and silver models.
The infuser should not be small to allow the leaves to unfurl and blend with the water, thus infusing their flavor. The agony of the leaves requires plenty of room, so a decent size infuser is important.