Cinder blocks are made with coal cinders or fly ash as the aggregate, which makes them about 25% lighter than standard concrete blocks. Modern "cinder blocks" are often actually lightweight concrete blocks made with expanded shale or pumice, but the term has stuck.
A standard 8 × 8 × 16 inch hollow cinder block weighs about 26–35 lb. The variation comes from the specific lightweight aggregate used by each manufacturer.
True cinder blocks (using actual coal cinders) are rarely produced today — they were phased out as coal-fired power plants closed. Most blocks sold as "cinder blocks" at home centers are lightweight CMUs.
Cinder blocks are weaker than standard concrete blocks and aren't allowed in load-bearing applications under most modern building codes. Use them only for non-structural projects like garden walls, raised beds, and decorative work.