Little quick background here.
The Unitarian Universalist Association of Congregations was incorporated in 1961. The new denomination came about by a merger of the American Unitarian Association and the Universalist Church of America. Currently, the UUA has in the neighborhood of a thousand congregations in the United States, and around 215,000 members of record.
Both the Unitarians and the Universalists were considered to be liberal Christians during the 19th century. Unitarians rejected the concept of the Trinity, and had a tendency to think that God would reveal himself in many different ways and many different places. Universalists, in direct opposition to the Calvinist doctrines of the total deparavity of humankind and the concept of savational election, proclaimed that all of creation - including all human beings - would eventually be reconciled to God.
While different in their beliefs, Unitarians and Universalists both believed that their faith was not of much value if it was not translated into action directed toward making the world a better place. The social justice movements of the 19th century saw both Unitarians and Universalists in the forefront of those movements.
The 1920's saw both movements impacted by the Humanist movement, which made a lot of sense when you consider that both Unitarians and Universalists had long been far more open to varying opinons on religious matters and more united on fighting social ills. During the remainder of the first half of the 20th century, the two denominations began to share resources. Finally, in the 1950's the process to merge into one body began and was completed in 1961.
Unitarian Universalists are congregational in their form of church government -that is, each congregation sets its own program, calls its own minister (or not - there are a number of very well run UU congregations that are lay led), and has its own unique spiritual flavor, based on the understandings represented by the local membership. This means that you may go to one UU church and feel almost as if you are in a liberal Protestant church. The next one may feel more like attending a top notch lecture series than a worship service. Still the next one may include elements such as a sage ceremony.
We UU's like to refer to ourselves as a "covenental" people rather than a "creedal" people. The closest thing we have to a denomination covenat would be our Seven Principles. Anyone who wants to review those in more detail may want to visit
Unitarian Universalist Association.
To sum up, Unitarian Universalism is a faith that is:
1. non-creedal
2. social justice oriented
3. congregational in basic organization
4. sees truth in many different faiths
5. covenental in its approach to life and living
That is most likely enough for now. If anyone has any specific questions, I will try to answer them to the best of my ability, or point you to resources on the web where you can find an answer.
Unitarian Universalism Definition 
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