Private Message. If you click on my name there's a "send private message to..." on there. It's like email except that it's through the system here. They don't let newbies PM people, so that might not even show up under my name as a choice, I don't know. I also don't know how many posts it takes to be allowed to do it. If it's like 10, you're close.
If your purpose, however, is getting private help, please note that they have a rule that things which can be posted on one of the forums stay on the forums. The PM should be reserved for something that is private info (like if you needed to give me sensitive info in order to answer a question), or for general chit chat or comments, that type of thing. They try to promote keeping it public where everyone can see and share.