At a basic level, we can classify systems into two types: simple and non-simple systems, depending on how many parts they have. Simple systems have very few parts that follow simple rules [5]. Examples include a pendulum, a lever, or a basic pulley system. With a few exceptions, the behavior and outcomes of these simple systems are easy to predict because they depend on just a few parts and their relationships.

Systems with many parts can be divided into two categories: complicated systems and complex systems. A complicated system consists of many interconnected parts, each with a specific function. The interactions between these parts are usually well-organized to achieve a particular goal. For example, an automobile comprises hundreds of different parts, all working together to make the car function as a vehicle. Similarly, many machines and engineered systems are complicated systems. Some natural systems are also complicated systems. For instance, the human digestive system comprises different organs that work together to break down food and absorb nutrients. Since the parts of a complicated system often have specific functions, removing one or more key parts can cause the system to stop working properly. If a car’s engine or battery breaks, it can no longer drive, and when someone has a stomach problem, it can lead to issues like indigestion.

Compared to complicated systems, complex systems often consist of many similar parts, such as cells, ants, molecules, people, or human organizations like companies. For example, a car has many different parts that each serve a specific function, but in an ecosystem, individuals of the same species are quite similar. 

The behaviors of the individuals (hereafter, the word "individuals" is used to mean the parts of a complex system) in complex systems are often broken down into a set of simple rules, such as alignment, cohesion, and separation for birds in a flock, or foraging, reproduction, death for living organisms within an ecosystem. These individuals interact with one another according to these simple rules and yield self-organized, unpredictable outcomes [7]. 

Since a complex system typically contains many individuals, these individuals make decisions or behave based on the "local" situation, including humans. Each one only directly interacts with the individuals within their reach at a given time. For example, a plant directly competes with the sunlight, water, and nutrients with the surrounding plants. A person normally carries out his/her daily routine to interact with some other people based on age, role, job, and experience. However, over time, each individual may impact all individuals indirectly because these interactions occur across the system simultaneously and continuously. 

In a complex system, the interactions between the individuals are not controlled by a single central leader or a key component. Instead, the system behavior results from continuous, simultaneous interactions among all individuals. This means complex systems can tolerate adding or removing some individuals, and some complex systems can adapt to these changes [5]. Many natural and human-organizational systems are complex systems, such as bee hives, ant colonies, and human communities. These systems remain functional even when individuals move in or out. Furthermore, disorder is an inherent aspect of complex systems. Therefore, the outcome of a complex system cannot be precisely predicted; the same inputs yield different results each time.