Rows are called periods.

Columns are called groups.

The type of block an element is located in matches the type of subshell that its outer shell electrons occupy. 

For example, all p-block elements have their outer shell electrons located in a p-subshell.

To leave space for elements that hadn't been discovered yet.

This ensured that elements with similar properties stayed in the same group.

1. He mainly ordered the elements by increasing atomic mass.
2. He grouped elements that had similar properties together.
3. He left gaps for elements yet to be discovered.

To group together the elements that have similar properties.

In order of increasing atomic (proton) number.

The group number tells you the number of electrons in the outer shell of an atom.

For example, all elements in group 3 will have 3 electrons in their outer shell. 

The period tells you the number of electron shells an atom has.

For example, all elements in period 2 have 2 electrons shells.

s-block, p-block and d-block.

There is also the f-block where the lanthanides and actinides are located.

The discovery of isotopes explained why some elements had different atomic masses but similar chemical properties. This validated Mendeleev's decision to prioritise grouping elements by their properties over strictly following atomic mass order.