An Introduction...
A simple, but accurate, description is that Freemasonry is two things...
A Way Of Life and a Moral Education
This brief introduction explains something about each of those aspects.

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A Way of Life

There are about 4 million Freemasons under more than 100 "regular" Grand Lodges worldwide.  Membership of the Craft is open to every respectable man who believes in his personal Supreme Being - usually known in Masonic terms as the Great Architect of the Universe, Grand Geometrician of the Universe or Most High.  These descriptive names show the importance the Freemason places on his Supreme Being in the order of things in Life.

Principles

Three great but simple principles, Brotherly Love, Relief and Truth, unite Freemasons of every color and many creeds.  Non-Freemasons often unfairly criticize not knowing that members of the Craft are guided by such honorable objectives.  The good Freemason builds these principles into his daily life as a law abiding Man and Citizen of the world.

BROTHERLY LOVE is the concern which each Freemason has for his Brother, which is readily shown by tolerance and respect for the beliefs, opinions and practices of his fellows and his willingness to care for his Brother and that Brother's dependants.

RELIEF The Freemason is by nature and teaching a charitable man.  He will cheerfully and kindly assist those less fortunate (whether Freemasons or not!).  He will care for and support his community - local, national and international.

TRUTH  The Freemason believes in Truth in all things in honesty and integrity in his personal, business and public life, in fair dealings and in firm standards of decency and morality,

An Education

As every man progresses in Life by education so every Freemason is taught how he can be a better Man.  his is done by a series of degrees - each degree educates him and answers some questions but leaves a door beyond.  When the candidate has grasped the teaching of one degree, that door is opened by his progress to the next degree.
Freemasonry is believed to have begun its evolution 500 or more years ago among the bands of working, skilled builders known as "masons".  The traditional framework into which most Masonic degrees are woven is the story and symbolism of King Solomon's Temple.  The connection of Masonic teachings to the building of that great edifice comprises a system of education related to the traditional craft of masons and interesting to the candidate - and therefore more likely to be effective in his development.

Freemasonry and Religion

A man's religion is precious and personal to him.  Discussion on matters of religion often causes arguments and many wars have been fought ostensibly to impose or defend one religion against another.  As he loves all his fellows, understandably Religion is a topic which the Freemason, in that capacity, is just not  allowed to discuss - nor would he want to.
Freemasonry is certainly not a Religion.  It has no "Masonic" God.  When Freemasons pray together, as they do in their lodges, each is addressing his personal Supreme Being.  So Freemasons of many creeds can in love and fellowship meet and pray together, irrespective of their individual religions.  The Holy Book (known as the Volume of the Sacred Law) of each religion represented in the Lodge is open during meetings.  It is common for 5 or 6 different V.S.L.'s to lie open during Lodge meetings abroad.

Freemasonry and Politics

For much the same reasons as above the discussion of political matters among Freemasons is absolutely prohibited.  A man's politics are his own concern and the Craft, being completely non-political, will never interfere in the world of Politics nor will the Grand Lodge express any views on political ideology or theories.

Freemasonry and Secrecy

It is often wrongly stated that Freemasonry is a Secret Society.  There are many thousands of books on Freemasonry openly available in libraries everywhere.  The Masonic Temple is usually a conspicuous building in most communities.  Many lodges advertise and publish details of their meetings in the local newspaper.  The various Museums and Libraries of Masonry are open to - and used by - members of the public who are not Freemasons.  Are these the hallmarks of a Secret Society?
The truth is that the principles and many of the practices of Freemasonry are anything but secret.  Members are perfectly free to make it known that they are Freemasons.  The only Masonic "secrets" are just those methods which members of various degrees throughout the world use to recognize and greet each other.  It's as simple as that!

Summary

Freemasonry is an ancient and honorable Society.  Its principles are just steady standards of life and conduct in a changing world.  The practice by the Freemason of Brotherly Love, Relief and Truth and the other principles of the Craft will go a long way to making a good man better.