In nature, micro-organisms are incredibly diverse, and consist of both useful and harmful types. Pasteurisation is a process used to considerably reduce the number of micro-organisms, to a level where it is unlikely that they are able to cause disease, while conserving the desired properties of the product. With sterilisation, on the other hand, the aim is to kill all the micro-organisms in the food or liquid; this process inevitably alters the desired properties of the product, although frequently it is the only way to be able to conserve foodstuffs for a number of years.
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