Cohesion
Cohesion is simply a measure of how similar functions in a group are. The greater the cohesion between two or more functions, the more likely they should be grouped together in decomposition results. This helps to ensure that similar functions are grouped together and helps to identify potential duplicate functions that may be performed by different groups, or at different steps in a sequence. If a function does several things that have low cohesion, there is a good chance that the function needs further analysis and may need to be broken into several discreet functions at the same level as the current function.
Coupling
Coupling is a measure of interdependence between two or more functions. So a change to a function or function group with low coupling would affect very few other functions outside of its function group.
The measure of a good decomposition effort is that the sub-units have high cohesion and high coupling only within themselves, and low cohesion and low coupling with any other sub-groups.