enhancement mainly deals with changes in program like say a new store is being added with some unique features, then all the related programs need to be enhanced to be applicable for that new store also.at the same time suppose you want to give some more facilty to the user then for that facily you need to enhance the program.Project realted to this type called enhancement type of project.
where as maintanance is if anything goes wrong with the existing program then chek them, fix the problem and again put it back for the purpose it is required.
You can say in this way:
Maintainence: Service Centers
Enhancement: Where new features are added in the same model, and it is being said as new car...