Unitarian Universalism is a liberal religious tradition. It's not a Christian tradition, though there are members who are Christian. In fact, you could say that about almost any major religious tradition in the world (though the percentages will naturally be smaller) - it's not an Islamic tradition, though there are members who are Muslim. It's not a Humanist tradition, though there are members who are Atheists and Humanists. It's not a Pagan tradition, though there are members who are Pagan.
What it is, is a way for people to come together and have a faith community without having to agree on the specifics. Unitarian Universalist tradition draws from a variety of sources (see here for more info), including "spiritual teachings of earth-centered traditions". It promotes seven basic principles, though UUs by no means have to agree with all of them - the last of these is "respect for the interdependent web of all existence". These Pagan principles were added in 1996, after a great deal of debate at the annual General Assembly.
I first began attending my local UU church for two reasons. First, I want my children to be raised and educated in a diverse religious background. They have about five different religions in their family anyway; I wanted a faith community that would grow and encourage these differences. I wanted somewhere with lots of kids, where they could go to Sunday school and feel 'normal', but not be subjected to "Jesus is the only way" propaganda. Second, I wanted a faith community for myself. I grew up in a larger church; and while Paganism in its many facets is amazing and I love it, there is a part of me that attends gatherings of 10-20 and wishes there was an entire congregation of people. I also really enjoy the interactions with different religions - I am somewhat of an amateur student myself, and I am always up for a good religious discussion!
I have found that both of my desires from the UU church are well and truly met. My local church doesn't have a CUUPS (Covenant of Unitarian Universalist Pagans) chapter, but honestly that's not what I'm there for. There are a few exclusively-Pagan groups around that I enjoy attending when I want to hang out with just Pagans. But I really enjoy the diversity and range of belief within Unitarian Universalism. It's kept me coming back for eight months now, and I think it will keep me coming long into the future.