There are numerous different types of systems models, ranging from the extremely simple to very complex. At the simple end are conceptual or flowchart models. These are simple depictions of a system, often simply drawn using pen and paper, which indicate which parts of the system are connected where.
At the other end of the spectrum are huge software models that simulate incredibly detailed processes at high speed. An example of these are the Global Climate Models (GCMs) which simulate the trajectory of the earth’s climate over the coming decades, used by the Intergovernmental Panel on Climate Change (IPCC) to prepare its periodic assessment reports on the current threat of climate change.