Classifying elements is necessary in order to determine their nature and the possible reactions.
We can classify them according to phases: Solid, liquid and gas. Most elements in the periodic table are solid at room temperature. Some metals are liquid at or near room temperature (Hg, Fr, Ga, Cs). Most of the non-metals are gases at room temperature.
We can also classify them as metals, non-metals or metalloids. Most of the elements are metals. Six (or eight if Po and At are included) of them are metalloids and 17 (some says 19) are non-metals.
Another way of classifying them is by periods, groups and blocks. Groups are the vertical columns in the periodic table. There are 18 groups in the table from alkali metals to the noble gases. Periods are the horizontal or rows across the periodic table. There are 7 periods in the table. Blocks represent the location of outermost electron of a particular element. Elements can be grouped as s, p, d and f blocks.