What is a System?
As we know that a system is set of elements which are not just interrelated, but also interconnected with each other to result in one integrated working body. It's a minimum unit consisting of various components functioning as a one.

What is a structure?
A structure is anything that is consisting of more than one element of a system arranged together in one way or the other. It's a framework that decides how the components are arranged or organized. While we know that system is a multiple parts of it working as a whole, but this wholeness needs to be composed and configured in some way, have some internal divisions and connection between these parts of the system.

Relationship between system and structure
While a system can be characterized by a structure, but a structure alone is not enough to describe a system. A system is much more deeper than a structure. A system can be vaguely seen as structure with some properties. Structure is more of a formal and abstract concept, while system is an implementation or realization of it.

Structure not only defines the position and movements of elements in a system, but also how they are sequenced. In other words, structure is set of rules and laws that determines what is the composition and functioning of a system apart from its properties and finally how stable the system is going to be.

Three types of structures
1) Unorganized Structure like a herd of cattle or a group of dissimilar things like hand full of sand, rock and dust.
2) Organized Structure like a molecule or an atom. They are relatively stable and their relationships are governed by some sort of law. Properties of these type of structures are not just the sum of the properties of their individual parts. For example, water can extinguish fire, but its parts hydrogen and oxygen are known to boost the fire combustion.
3) Organic structure like a living organism, an art or a society. Self development and self reproduction is their main feature.