Exorcism

Exorcism is the act driving out supposed evil spirits from the body of one that such spirit had possessed.
We have come to know exorcism in a common term "deliverance"
However, that definition only just makes it a simple act, undermining the effort taken to achieve this herculean and usually life threatening process.

What is exorcism?

Exorcism (from Greek εξορκισμός, exorkismós "binding by oath") is the religious or spiritual practice of evicting demons or other spiritual entities from a person, or an area, that are believed to have possessed. Depending on the spiritual beliefs of the exorcist, this may be done by causing the entity to swear an oath, performing an elaborate ritual, or simply by commanding it to depart in the name of a higher power. The practice is ancient and part of the belief system of many cultures and religions.
Link

Exorcism cuts across all religions. In christianity, it's majorly termed "deliverance" , in islam it's called "Ruqyah" even the buddhist teachings show such evidence.

So, do the Thervada monks, they recite some verses to get rid of the ghosts. The Mahavamsa, the ancient chronicle of Sri Lanka also mentions these ghosts that were exorcised by Buddhism
Link

So...what are we exorcising and how did they get there?
According to the definition above, what is usually exorcised are spiritual entities. However, these range from malevolent spirits of some sort. Some believe that spirit of dead people take possession of living bodies but usually this isn't the case. Books like ; the devil's new biography gives insights about how these gruesome acts which are seen or portrayed in movies in seemingly theatrically intriguing and exciting ways. But the actual thing is, it's far more dangerous and far scarier than it looks. So how does one get possessed?

Malevolent spirits inhabit unhealthy places. It will be observed that, most of such places where these spirits are found are: abandoned structures, slaughter houses, dumpsites, putrid shrines and surprisingly music stores.

The nature of demons according to the book "the devil's new biography" is such that is capable of diffusing between the layers of the human skin hence allowing possession to occur.
There are so many symptoms of possession, but this video might just do justice to that.

Link