What is Function group? Difference between function group and function module?

Function Groups act as containers for Function Modules that logically belong together.

Function Groups
1) These cannot be defined in a Function Module.
2) It cannot be called.
3) They are containers for Function Module.

Function Modules
1) These must be defined in a Function Group.
2) It can be called from any program.
3) They are not containers for Function Group.