What is the difference between 2D and 2 1/2 D memory organization?

In 2D organization the cells form one array of rows and columns. A single decoder takes all $n$ address bits and drives one of $2^n$ word lines, so a whole word is selected at once. The decoder becomes very large for big memories.

In 2 1/2D organization the address is split into a row part and a column part. Two small decoders (row and column) select the cell, so each needs only about $2^{n/2}$ outputs. The total decoder size is much smaller, which makes it cheaper and the usual choice in real chips.