The elements in a single vertical column are referred to as members of a group or a family. These atoms are similar and have identical or very similar outer configurations. A horizontal row in the periodic table is called a period and these correspond to the highest occupied electron level, hence for H and He, n = 1. The 4th period became longer as electrons enter d subshells. The seventh period includes most of the man-made radioactive elements.

After the 6s subshell is filled in barium then the third transition series begins at lanthanum (Z = 57). But, the next electrons fill the 4f subshell (begginning with cerium, Z = 58) to give the first f-transition series, called the lanthanides (Ce to Lu).

The elements from Th (Z = 90) to Lr (Z = 103), in which the 5f subshell is filling, are called the actinides.

Each period ends with a noble gas - an element in which all energy subshells that are occupied are completely filled. This allows us to write the electron configurations of atoms as such:-For example: 

Information provided by: http://www.bpx.cov.ac.uk