What is molecular system in general terms? What is a supramolecular system?
Molecular chemistry is based predominantly on interactions through covalent bonds. It looks at molecules as individual entities, and is limited in its scope of accomodating intermolecular interactions. Your solution of sugar in water could easily be studied as a molecular system, if all you are interested in is the sugar molecule itself. Subtle differences in molecular conformation of the sugar in different solvent systems, for instance, would simply be written off as 'solvent effects'. 

Supramolecular chemistry is based on intermolecular interactions. Typically, these are based on weak interactions (non-covalent forces), and include:


ion-ion interactions
ion-dipole interactions
dipole-dipole interactions
hydrogen bonding
van der Waals forces
pi stacking
hydrophobic interactions
and probably some others


It is through these types of interactions of that we can explore phenomena such as host-guest association, self-assembly, metal-ligand chelation, micelles, and even the structure of DNA, as all of these systems rely on the weak intermolecular interactions outlined above. You could also study your aqueous sugar system as a supramolecular system, and here you could consider what predominant intermolecular interactions are at work, and how they affect the overall chemistry of the bulk system.

Water is supposed to exist as water clusters, weak oligomeric and polymeric forms of $\ce{(H2O)_{n}}$