Micro-organisms (or microbes, or even germs) are just that ... micro-sized.
For a long time in human history, their existence was never even suspected: for example, the great typhoid and cholera epidemics in cities like London in Victorian times were put down to "something in the nasty smells in the air". So people would walk around with sweet smelling things under their noses to avoid becoming ill.
Now we know a lot more about them and this worksheet acts as a simple introduction to the world of microbes.