Ceramic water filters are an inexpensive and effective type of water filter, that rely on the small pore size of ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 material to filter dirt, debris, and bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 out of water.
As with most filtration methods, water is carefully introduced to one side of the filter, which acts to block the passage of anything larger than the pore size. Typically bacteria, protozoa, and microbial cysts are removed but the filters are not effective against viruses since they are small enough to pass through to the other "clean" side of the filter.