The 'F' is a rendition of the Christian cross. The 'O' is Buddhist dharma wheel. The 'N' is the Hindu aum symbol. And the yin and yang is actually derived from Taoism, which is a form of religion, although a lot of people use the meaning of the symbol as more of a philosophical mindset.
Hope this helps.
Thats what I have on my leg. I love getting asked what it says when people see it. Being in Florida, I'm in shorts a lot so I get asked quite a bit. Its always fun telling people what it says and seeing their reactions. And on the rare case that I actually meet a fellow atheist and they ask about it, its very cool talking with them about it. By far my favorite tattoo that I have!