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.
Of course, the reason that no one knew about microbes then is that they are too small to be seen with the naked eye. Things like bacteria and viruses are incredibly tiny. Not all of them are harmful, and cause disease; in fact, there are many bacteria that are really helpful, breaking down organic material to form humus in the soil, helping animals to digest their food, that sort of thing.
Now we know a lot more about them and this worksheet acts as a simple introduction to the world of microbes.