Any destructive living being that creates nuisance in human lives and disturbs it in any form is known as pest. Examples can be as growth of algae and fungi on walls near water, breeding of rats in an old cooler kept inside the store room, squirrel making home in ventilator area, cat living permanently in the kitchen area, crows eating all the crops in the fields.
In all, these creatures are living their own lives but their habitat is getting merged with human habitat in excess. This can give rise to diseases, lack of space, dirt etc. In such a situation, an expert help from Pest control services is needed.