A functional group is a portion of a molecule that is a recognizable/classified group of bound atoms. In organic chemistry it is very common to see molecules comprised mainly of a carbon backbone with functional groups attached to the chain. The functional group gives the molecule its properties, regardless of what molecule contains. they are centers of chemical reactivity. The functional groups within a molecule need to be identified when naming.
Related Journals of Functional Group
Biochemistry & Analytical Biochemistry, Journal of Organic & Inorganic Chemistry, Modern Chemistry & Applications
The functional group approach "works" because the properties and reaction chemistry of a particular functional group (FG) can be remarkably independent of environment.