Gulliver represents a middle-class Englishman who is decent and well-intentioned. In the course of his travels, he becomes less tolerant and more judgmental of the nations he visits.
The Lilliputians, a tiny race of people, represent much of what is petty and small-minded about the English and humankind in general. They are physically and morally smaller than Gulliver. They are self-important, self-serving, hypocritical, and dangerous and cruel in spite of their small size.
The Brobingnagians, the race of giants, are physically and morally bigger than Gulliver. They have not built vice into their government and institutions. Therefore, they represent much of what is good in humankind. However, the great size of the Brobingnagians means that Gulliver can never feel safe or equal in their society; while they treat him kindly, they also treat him as a an exhibit.
The Laputans represent the dangers and limitations of abstract and theoretical knowledge.
Their addiction to abstract knowledge make them oblivious to each other and to all human concerns.
The Houyhnhnms represent reason and virtue. They concern to the good of their society as a whole. So deep-rooted is this tendency that they have no distinguishing characteristics or names, and they do not seem to possess an emotional life beyond treating everyone with respect and kindness.
The humanoid Yahoos represent all that is bestial, low, and despicable in human behavior. Gulliver is ashamed to recognize the similarities between them and human beings, including himself. They are greedy, violent, dirty, and destructive of themselves and others.
At the end, humans are better than Yahoos, but hey are also worse, since humans have the ability to choose between good or evil, and frequently choose evil. 

Finally there are recognizable elements of political allegory through allusions to people and events in the England of Queen Anne and George First.

The traveller usually discovered some happy society where men lived a simple, uncorrupted life, following natural instinct and the innate light of reason; and from these utopias European man was seen as the victim of civilization.

Gulliver’s experiences are different because the people among whom he is cast are in no sense children of nature.

They all live in highly organized societies and are governed by institutions.

If in the end he takes an aversion to everything at home, it’s because Europe is losing its civilization and falling into a state of corruption, expressed in the novel by the constant opposition between rationality and animality.

Swift’s main technique for achieving this is the basic plot of science fiction: the voyage by an average civilized human being into an unknown land and his return home.

This opens up a lot of satiric possibilities, because it enables the writer to play off three different perspectives in order to give the reader a comic sense of what is very familiar.

The key to this technique is generally the use of the traveller, the figure who is the reader’s contemporary and fellow countryman.

The hero’s reactions to the New World of utopian perfection can be a constant source of amusement and satiric comment.

The genius of Swift’s satire in “Gulliver’s Travel” realizes itself in a second feature, the way he organizes the New World by constantly changing the perspective on human conduct; with this altered perspective, Swift can now manupulate Gulliver’s reactions to the changing circumstances in order to underscore his satiric points in a very humorous way.

But the most common interpretation among Swift’s contemporaries was that Swift’s main satiric point was to ridicule the Europeans’ pretensions to rationality; many who saw this in the satire simply dismissed it as mistaken vision since they believed that the promises of the new science were being realized and that Swift was simply a pessimistic.

However, this reaction acknowledged that Swift had a seriuos purpose and that Gulliver was his spokesman.

A second interpretation claimed that Swift was mad, mentally unbalanced, and therefore the reader doesn’t need to consider the ending of the book seriously.

In the 20th century, criticism held that we aren’t supposed to see Gulliver’s actions as the natural rational result of his experiences, because Gulliver himself has finally become the target of the author’s satire.

Gulliver no longer speaks for the author, but his dislike of the Yahoos and his conduct are a sort of warning for us.

This approach maintains the claim that Swift was a cleaver writer, in control of his literary medium.

 

Throughout Gulliver's Travels the narrator spends a great deal of time discussing the human body-going so far as to detail his own urination and defecation. In each of the various lands to which Gulliver travels, he comes face to face with excrement.. By looking closely at the body as a material thing and paying attention to what humans do on a daily basis, Swift makes it impossible to look at humans as exclusively spiritual or intellectual beings.

Gulliver is a reader: He reads whenever he has the time. on each of the islands he visits, he makes a point of noticing whether the inhabitants write or do not write. 

Gulliver is also a master linguist, On each of the islands he visits, he learns the language quickly. His ability to communicate suggests the value of communication across cultures. 

Gulliver comes into contact with several different races of people, all of which are narrow-minded in some way. Much of Swift's satirical focus is on people who cannot see past their own ways, their own power, or their own beliefs. 

In Gulliver's Travels the reader comes to realize that much in the world really is relative. Gulliver's first journey lands him in Lilliput where he is called the Mountain Man, because the people there are only five to six inches tall. On the other hand, in Brobdingnag, Gulliver is tiny compared to the enormous creatures who find him and keep him as a pet.

Many other classic works use the theme of the travel, such as Chaucer's Canterbury Tales and Homer's Odyssey. Travel in the case of Gulliver's Travels gives Swift the opportunity to compare the ways of humanity, more specifically those of the English, with several other ways of living.