the holes in ceramic tea strainers are larger than with other types of strainers so for certain types of tea, the leaves/herbs can slip through into your beverage. if you can deal with the occasional leaf in your cup, they work perfectly fine.
Ceramic tea strainers are made so that part of the ceramic is porous. The infused tea can pass through, but the leaves remain behind.
Very pleasant and easy to use, but difficult to clean - use a solution of baking soda.
I've only seen them used in China. The ceramic strainers rest in the tops of ceramic teapots and mugs. The strainer is placed in the recptacle. A small amount of tea is placed in the strainer. Hot water is poured over the tea, filling the mug or teapot. The tea in the strainer can be reused.