Groups - vertical columns of elements. Elements in group X will have X electrons in their outer shell of electrons. It is this shell which is involved in chemical reactions.
For example, lithium is in group 1, and therefore has 1 electron in its outer shell. This affects its chemical reactions, and gives it a valency (or combining power) of 1.
This means that elements in the same group have similar chemical properties, as all elements in the group need to lose/gain the same number of electrons in reactions to achieve stable electronic configurations and so will react with the same elements in a similar manner.
Periods - horizontal rows of elements. Elements in period Y will have Y shells of electrons. This affects the properties of the element.
For example, bromine is in period 4, and so has 4 shells of electrons. It has different properties to chlorine, in period 3 (namely it has a higher boiling point).
For example, lithium is in group 1, and therefore has 1 electron in its outer shell. This affects its chemical reactions, and gives it a valency (or combining power) of 1.
This means that elements in the same group have similar chemical properties, as all elements in the group need to lose/gain the same number of electrons in reactions to achieve stable electronic configurations and so will react with the same elements in a similar manner.
Periods - horizontal rows of elements. Elements in period Y will have Y shells of electrons. This affects the properties of the element.
For example, bromine is in period 4, and so has 4 shells of electrons. It has different properties to chlorine, in period 3 (namely it has a higher boiling point).