Chemical properties of an organic molecule is determined not by the whole molecule but by a specific region in it, called the functional group of the molecule.

eg. 1:
Compounds 1 and 2 have similar chemical properties, i.e., they react with the same reagents under similar conditions to give similar products.  The reason is the COOH group is common to both 1 and 2.  That is, the COOH group, called the carboxylic acid group, is the functional group in 1 and 2.

eg. 2:
Compounds 3 and 4 have similar chemical properties because CHO group is common to both 3 and 4. That is, the CHO group, called the aldehyde group, is the functional group in 3 and 4.

Organic Compounds are classified based on the functional group present.  Compounds 1 and 2, which contain a carboxylic acid group as the functional group, are called carboxylic acids.  Compounds 3 and 4, which contain an aldehyde group as the functional group are called aldehydes.