Hi Smurf, yeah am afraid there is a difference... well in most instances anyway.
daycream - To protect the skin from the harmful surroundings by producing a barrier. I.e sun, pollution and leaves it radient and soft. i was once told its like keeping a sandwich in clingfilm, keeps it fresh (if you dont use the sandwich goes all dried and horrible)
Nightcream - Has all the nutrients and actually does the work! i.e anti aging... firming, putting nutrients into the skin (hense its richer!)
I am sure they now do allrounders, but thats the general idea anyway.
Hope that helps