Can someone explain Credit Karma in a nutshell?

Hello,

In glancing at a thread about a CC, I noticed that a bunch of people listed their credit scores and what they got approved for. I made a lot of mistakes when I was young and dumb, so I know my credit isn't good - but seeing the scores made me wonder where I am. I paid for my score from one bureau from MyFico.com. . . but then I noticed everyone mentioning their scores from CK, which I realized meant Credit Karma. From what I can gather, the score they give isn't anything official, but I'm curious about how it might relate to the score I purchased. However, I don't really understand how they operate. How do they get my information for free? Does obtaining a score from them give my credit reporta hit?

The main good thing on my FICO score was that I haven't had any inquires in the past year. Since my score is pretty low, I don't want to do anything to drop it - hence my concern about credit karma.

Also, since I currently have no credit cards, the my Fico report suggested that I get one (and obviously be responsible with it) to boost my score. My girlfriend and I are contemplating buying a house in five or six months (more of a long term partnership than a fleeting relationship). She has absolutely excellent credit. I know my credit score isn't going to help, so I'm trying to think ahead and do what I can to improve it. I have no current need for credit - and our house timeline is very fluid - but like I said, I'm looking to do what I can. I figure that part of that is arming myself with information.

So how does Credit Karma work and does it make sense for me to check and compare?

Thanks for your wisdom.