The shape can vary. Many are 8-sided as a reminder of the "new creation" and as a connection to the practice of circumcision which traditionally occurs on the 8th day. Some are 3-sided as a reminder of the Holy Trinity Father, Son, and Holy Ghost. They are often placed at or near the entrance to a church's nave to remind believers of their baptism as they enter the church to worship, since the rite of baptism served as their initiation into the Church.
The quantity of water is usually small (usually a litre or two). There are some fonts where water pumps, a natural spring, or gravity keeps the water moving to mimic the moving waters of a stream. This visual and audible image communicates a "living waters" aspect of baptism.
The mode of a baptism at a font is usually one of sprinkling, pouring, washing, or dipping in keeping with the Koine Greek verb βαπτιζω. Βαπτιζω can also mean immerse, but most fonts are too small for that application. Some fonts are large enough to allow the immersion of infants, however.
Some church bodies use special "holy water" while others will use water straight out of the tap to fill the font. A special silver vessel called a Ewer can be used to fill the font. In many Eastern Orthodox churches, holy water is consecrated once each year in a baptismal font during the Feast of Theophany.
A baptismal font differs from an immersion tank. One may use a tank, pool, river, or lake for full-immersion baptisms where the person is fully immersed, dunked, or submerged under the water. This practice symbolizes the drowning of the old nature as found in Romans 6:3-4.