Science starts with observation. Scientists do experiments or observe natural processes and then try to explain why or how what they are seeing makes sense. Models are an integral part of this process of explanation. Scientists in the 19th century observed that electrons in atoms emit electromagnetic radiation. The puzzle was why only some selected frequencies of radiation were emitted and not others. The scientist Niels Bohr came up with a model of the atom in which electrons orbit around the nucleus but only in some special orbits. He used this to show why only specific frequencies of radiation are emitted.
Prediction is the flip side of explanation. If a scientist can build a model that explains a natural process well enough, she should be able to use the model to predict how the process will unfold in the future. A common example of model-based prediction is weather forecasting. Atmospheric processes depend on so many variables that they are exceedingly difficult to model. Scientists have valiantly tackled this task, however, and come up with numerical computer-based models that let them predict what the weather will be like a day or a week from now with some degree of success. A system called numerical weather prediction (NWP) is used by many forecasters today.
Physical and computational models can be built to replicate a natural process so scientists can study it closely under controlled conditions. One example is the study of wind erosion in a wind tunnel. Scientists simulate wind storms using large fans to study the effect they have on different kinds of soil. The wind tunnel can be used to test out different farming practices that could be used to keep soil from blowing away.
Models are indispensable for teaching science. A three-dimensional model of the solar system or the human brain can clearly convey information that it would be very difficult to communicate in words. More abstract models are used in teaching as well. A diagram or a flow chart outlining a process can be a very effective tool for teaching. The ubiquity of computers have brought sophisticated electronic models within the reach of today's school and college students.