The LEGO Brick is a brick made by the LEGO Company, in case the name didn't give that away. Specially designed for optimal pain when stepped on, Ole Kirk Christiansen eventually realized that his new invention, nicknamed the "GOOD GOD THE PAIN WHAT THE HELL DID I JUST STEP ON?", wasn't very popular with consumers and remarketed the bricks as a children's toy.
Bricks have cylinders on their top, called "studs" (yeah, no joke to make here...), and spaces at the bottom for the studs to attach to, binding the bricks together as the studs and holes exert pressure on one another. These bricks are also commonly, yet incorrectly, called "blocks" and "legos". Most sane people will not mind this. However, we are obligated to warn you that if you use either of these incorrect terms in front of a LEGO fan, he will hunt you down. And he will show you no mercy.