what are ortho -para directing and meta directin grops???

In benzene all the six hydrogens are equivalent,so any of the six positions can be occupied when monosubstituted products are formed.But when such products are converted into disubstituted ones,the incoming group are directed to a particular position by the substituents already present in the ring.This is called as directive influence of the group.Depending on their directive influence, various groups are divided into two categories:-
1.ortho and para directing groups- The substituents or groups which direct the incoming group to ortho(1,2) and para(1,6) positions are called ortho and para directing groups.For ex- -CH₃,-C₂H₅,-Cl,-OH,-NH_2,-OCH₃,-NHCOCH₃ etc.
2.meta directing groups- Those groups which direct the incoming group to meta(1,3) position are called meta directing groups.For ex- -NO₂,-CN,-CHO,-COOH,-COOR,-SO₃H etc,