Bricks are made with the two most abundant materials on Earth: clay and shale. These two materials are then put in a furnace, called a kiln, and heated to 2,000°F (approximately 1,100°C). By a chemical process (vitrification), the materials fuse together and form bricks.
Concrete bricks, gray bricks similar to clay bricks, are not really bricks, even though people call them such on a regular basis. Concrete bricks are significantly easier to break that clay bricks.
Due to the longevity of clay bricks, some of the American colonial buildings still stand today. A few examples are Virginia's St. Luke's Church (est. 1632), the Boston State House (circa 1713), and Philadelphia's Independence Hall.
Molds are used to keep the bricks a uniform size as they dry. The molds are bigger than the final size due to the shrinkage that occurs during the drying process. This is important when the bricks are used in construction to make sure that the right amount of materials is bought. It also affects the final appearance of any project if the bricks are not all the same size.