Bacteria are single-celled creatures with no nucleus. They are very small. They grow all over the Earth, in the ground, in the water, and even in our bodies. Since they can be found almost everywhere on this planet, from saline areas to hot springs to human colon, they are called omnipresent.
Some bacteria can cause diseases, but most do good things like break down waste and make oxygen, fix nitrogen so that plants can absorb it and synthesise their own food.
Harmful bacteria are called gram positive bacteria.