A columnar DBMS is an implementation of the relational theory, but with a twist. The data storage layer does not contain records. It contains a grouping of columns.
Due to the variable column lengths within a row, a small column with low cardinality, or variability of values, may reside completely within one block while another column with high cardinality and longer length may take a thousand blocks. In columnar, all the same data - your data - is there. It's just organized differently (automatically, by the DBMS).