A block wall and block column under construction. Image courtesy of Gabric Construction.
Masonry blocks are shells made of aerated concrete which are stacked and set into place with mortar. These shells are often filled with concrete grout and vertical and horizontal steel reinforcement bars to greatly increase the strength of the wall.
In this way, masonry block walls are a cost-effective alternative to concrete walls which require careful erection of plywood formwork, bracing of the form work, and more steel reinforcement to prevent cracking.