Main part of the engine that houses all the reciprocating components which create the power. It's comprised of the cylinder bores where the pistons slide up & down. The number of cylinders an engine has refers to the number of cylinder bores & pistons.
The cylinder head sits on-top the block, protecting the pistons & internal components. Depending upon the configuration of the engine block, it may have multiple heads.
Engine blocks are typically made from cast iron or cast aluminum with reinforced cylinder sleeves / bores.